Diversity of Microbes and Cryptogames
Bacteria
Dr. A. K. Paul
August, 2006
English
Contents
Introduction Morphology
Nutrition and Growth Of Bacteria Bacterial Reproduction
Economic Importance of Bacteria A General Account of Cyanobacteria
Significant Keywords:
Bacteria, Peptidoglycan, Capsule, Flagella, Binary fission, Endospore, Cyanobacteria, Antibiotics, Bioremediation
Introduction
Bacteria are a heterogenous group of single celled prokaryotic microscopic organisms characterized by the lack of a membrane-bound nucleus and membrane-bound cell organelles, like mitochondria, plastids etc. They represent the first living inhabitants of the earth and evidence indicates that they existed more than 3.5 billion years ago i.e. almost 3 billion years before the appearance of plants and animals. Earlier they were considered as part of the plant kingdom; eventually they were separated into a kingdom called Protista by Ernst Haeckel (1866), which included microorganisms sharing characteristics of both plants and animals. Later, Robert H. Whittaker (1969) in his five-kingdom classification of organisms placed them in the kingdom Monera. Carl Woese and his co-workers (1977) in their three-domain system of classification divided the living organisms into three domains the Archaea (archaebacteria), the Bacteria (eubacteria), and the Eukarya (eukaryotes). Both Archaebacteria (the primitive bacteria) and Eubacteria (true bacteria) have prokaryotic cells.
Therefore, it is apparent that all organisms with prokaryotic cells are bacteria and conversely, all bacteria are prokaryotes.
The archaebacteria are a group of bacteria that live in unusually harsh environments where no other living organisms can survive and probably represent the first forms of life. They are found in extreme environments such as acidic hot springs, deep-sea hydrothermal vents, in arctic ice and glaciers and highly salty water. Archaea are structurally and chemically distinct from other bacteria. The cell walls, cell membranes, and ribosomal RNA are different from those of other bacteria.
The eubacteria are ubiquitious (omni presence) in nature, mostly occur as free-living organisms in the air, water and soil including quite a variety of natural and manmade environments. They are of great importance both in nature and in industry being involved in recycling of wastes and production of industrial products. Eubacteria are associated with human body as normal microflora, many of which are beneficial, while others are pathogenic causing diseases of different types. Bacteria have also associated with plants as symbionts and plant pathogens. This chapter discusses the gross morphology, ultrastructure, growth and economic importance of bacteria including cyanobacteria.
Morphology Size:
Bacterial cells are extremely small in size and could be visualized with the help of microscope at a magnification of 1000 times. They vary in size depending on the species and range from 0.5 – 1.0 µm in diameter (micrometer, µm = 10-6 m) in spherical bacteria like Streptococcus and Stapylococcus while, the rod shaped bacteria are usually 0.5-1.0 µm wide and 1-10 µm long. For example the rod shaped cells ranges from long Bacillus anthracis (1.0 to 1.3 µm X 3 to 10 µm) to very small cells such as Pasteurella tularensis (0.2 X 0.2 to 0.7 µm). Mycoplasmas or pleuropneumonia like organisms (atypical pneumonia group) are very thin measuring 0.1 to 0.2 µm in diameter. These bacteria could pass through the bacterial filters but rests are retained on such filters. In addition, there are few exceptionally large
bacterial species such as Epulopiscium fishelsoni (80 µm X 600 µm ) which grows in the intestine of Surgeonfish from Red Sea and Thiomargarita namibiensis (100 – 750 µm in diameter ) isolated from Namibia coast because of the small size, the ratio of surface area to volume for bacteria is very high compared to eukaryotic organisms with larger cells. This high surface/volume ratio facilitates absorption of nutrients and gases from the environment leading to high rate of metabolism and growth of bacteria. Moreover, the small size also helps them to spread rapidly in the environment.
Shape: Bacteria have three characteristic shapes: coccus (pl. cocci), bacillus (pl. bacilli), and spirillum (pl. spirilla). The arrangement of cells is also typical of various species or groups of bacteria (Fig. 1). The spherical or ovoid cocci have several arrangements depending on the planes of division. Division in one plane produces either a diplococcus (a pair of cocci, e.g.
Neisseria) or streptococcus (a chain of cocci, e.g. Streptococcus) arrangement. Division in two planes produces a tetrad (square of 4 cocci) arrangement, while division in three planes produces a sarcina (cube of 8 cocci). Division in random planes produces a staphylococcus arrangement where cocci are arranged in irregular groups or grape-like clusters (e.g.
Staphylococcus).
Bacilli are rod-shaped bacteria. Bacilli all divide in one plane producing a bacillus (a single bacillus), streptobacillus (a chain of bacillus), or coccobacillus (oval, almost similar to coccus). The curved rods or comma-shaped cells are called vibrios (e.g. Vibrio cholerae).
C D A B
Plate 1. Photomicrographs showing morphological forms of bacteria. (A) Staphylococcus, cocci in groups, (B) Ralstonia, short rods, (C) Bacillus, typical rod shaped bacteria, (D) Streptomyces, filamentous bacteria with chain of spores (indicated by arrow).
Figure 1 . Characteristic shapes and arrangements of bacterial cells
Spirilla, the spiral or corkscrew shaped bacteria come in two forms, a spirillum with rigid wall (e.g. Spirillum), or may be thin, flexible as in a spirochete. Many species of bacteria characteristically show the tendency of cell branching as in Mycobacterium, while in streptomycetes (e.g. Streptomyces, Actinomyces) there is formation of branched mycelia similar to those in fungi. In addition, bacteria of unusual shapes like square, star-shaped, spindle-shaped and lobed structure have also been discovered recently. The characteristic shapes of bacterial species are determined by the genetic makeup of the organism and most bacteria maintain a single shape and are called monomorphic, while pleomorphic bacteria alter their shapes and occur in different forms. Cyanobacteria or blue-green algae also show wide diversity in their morphological forms, which has been discussed in details under section Cyanobacteria.
Surface Structures
Bacteria have number of surface structures that are broadly distinguished as (1) surface appendages and (2) surface layers. Two types of surface appendages have been recognized in bacterial cells. These include flagella (singular flagellum), the organs of locomotion and pili (singular pilus) or fimbriae (singular fimbria). The surface layers include glycocalyx (capsules and slime layers) and S-layers.
Flagella: Structurally, bacterial flagella are thin, hairlike, filamentous surface appendages with helical shape that imparts motility to the bacterial cell. They are extremely thin and cannot be seen directly under the ordinary light microscope unless they are suitably stained (Leifson’s flagella stain) by layering a dye precipitate on its surface. However, they are easily detected under electron microscope. Bacterial flagella are about 15 to 20 µm long and about 12 to 20 nm (nm = 10-9 m) in diameter.
The number and distribution of flagella on the bacterial surface are characteristic for a given species and hence are useful in identifying and classifying bacteria. Cocci rarely have flagella, but in rod-shaped bacteria, flagella may be polar or lateral. Polar flagellation may be monopolar or bipolar. In monopolar flagellation, some bacteria like Vibrio and Caulobacter have only a single flagellum called monotrichous, while others have a cluster of flagella at one pole and are called polytrichous also referred to as lophotrichous (e.g. Pseudomonas and Chromatium). Bipolar polytrichous flagellation is called amphitrichous (e.g. Spirillum). In lateral flagellation the flagella may be inserted on lateral walls of the bacterial cell as in Selonomonas ruminatum or over the entire cell surface and is called peritrichous (e.g.
Bacillus, Proteus etc.) (Figure 2). As an exception, flagella in members of Spirochaetales are internal i.e. they remain beneath the outer membrane and wrap around the cell body. They are commonly termed as endoflagella or axial filament.
Figure 2 . Flagellation pattern in of bacteria (a) Monotrichous (b) Polytrichous (c) Amphitrichous (d) Peritrichous
A B
Plate 2. Electronmicrographs showing bacterial flagella. (A) Peritrichous flagella in Bacillus (B) Monotrichous flagella in Rhodopseudomonas.
Fine structure analysis following electron microscopic studies has subdivided the bacterial flagellum into three morphologically and chemically distinguishable parts designated as (i) filament, (ii) hook and (iii) basal body (Figure 3).
Figure 3 . Ultrastructure of bacterial flagellum
Filament: The section of the flagellum between hook and distal end which lies external to the cell surface is known as the filament. The filament is around 20 nm in diameter, 10-20 µm in length and wavy in nature with constant amplitude. In some bacteria like Bdellovibrio and Vibrio cholerae, a sheath surrounds the flagellar filament. Chemically, flagella are constructed of a class of low molecular weight proteins called flagellins, which are characteristic of a given species of bacterium and are arranged in helical order around an axial cylinder. During synthesis of the flagellar filament, flagellin molecules synthesized in the cytoplasm are added to the growing tip of the filament increasing its length.
Hook: The section of flagellum between filament and basal structure is known as the hook. It is short, slightly curved, and has a diameter somewhat greater than the filament. The hook ranges from 70-90 nm in length and consists principally of single polypeptide.
Basal body: The most complex part of the flagella, which anchors the flagellum in the cytoplasmic membrane and the cell wall. It consists of a small central rod and a series of discs or rings. The basal body acts as a motor system, enabling the flagellum to rotate and propel the bacterium in the liquid environment.
Function: Flagella, the organelles of locomotion impart motility by rotating in clockwise and anticlockwise manner controlled by the basal body. The movement of the basal body is driven by a proton motive force rather than by ATP directly. Bacteria swim through liquid by means of the propeller-like action of the flagella in response to environmental stimulus. The stimulus may be due to chemicals (chemotaxis), light (phototaxis), osmotic pressure (osmotaxis), oxygen (aerotaxis), and temperature (thermotaxis). Chemotaxis, referred to as movement in response to attractant and repellent substances in the environment help bacterial pathogens to move through the mucous layer and colonize the mucous membranes and thereby facilitate bacterial pathogenesis.
Pili (Fimbriae): Pili (singular, pilus) are thin, hairlike, hollow proteinaceous appendages on the surface of many Gram-negative bacteria. They originate from the cytoplasmic membrane and occur profusely on the cell surface. Structurally they are very simple, 3-10 nm in diameter and can only be seen by electron microscope. The pilus has a shaft composed of a protein called pilin and at the end of the shaft there is an adhesive tip structure.
Based on their morphology and physiological function the pili are distinguished into two distinct types: i) the common or attachment pili, also known as fimbrae are short and abundant and ii) Sex pili or F pili, also know as conjugation pili, are long and very few in number.
The common pili or fimbriae confer adhesive properties on the bacterial cells and allow the bacteria to adhere and colonize environmental surfaces or cells and are referred as colonizing factors. The sex pili are involved in sexual reproduction. They attach male to female bacteria during conjugation and help in the transfer of genetic material from the donor to the recipient cell.
Surface Layers
Glycocalyx: Some bacteria are surrounded by layers of viscous or amorphous substances called glycocalyx. These surface layers can be detected by staining and light microscopy;
electron microscopy of thin-sectioned cells, freeze-fractured, and negatively stained (with indian ink) cells. They are distinguished into two types: i) capsule and ii) slime layer.
In some bacteria like Streptococcus pneumonia, Leuconostoc mesenteroides and Xanthomonas campestries the glycocalyx appear as thick outermost gelatinous covering tightly bound to the cell wall and is called a capsule. Capsules may be up to 10 µm thick and
are distinctly visible under light microscope. The glycocalyx when remain loosely adhered to the cell wall in an unorganized manner it is commonly referred to as a slime layer. Slime layers commonly diffuse into the medium when the organisms are grown in liquid medium.
The glycocalyx is not essential for viability since viability of cells is not affected when capsular polysaccharides are removed enzymatically from the cell surface. The exact functions of capsules are not fully understood. However, they have been found to protect the cells against drying or desiccation, help in traping of nutrients and enable some bacteria to adhere to environmental surfaces like, rocks, root hairs, teeth, etc. and colonize to form a biofilm consisting of layers of bacterial populations adhering to host cells and embedded in a common capsular mass. As an example, Streptococcus mutans, a bacterium responsible for initiating dental caries adhere to the enamel of the tooth and form plaque. The glycocalyx enables bacteria to confer resistance to phagocytosis and hence provide the bacterial cell with protection against host defenses to invasion.
The composition of glycocalyx varies with bacterial species and the conditions under which it is grown (Table 1). Most capsules consist of polysaccharide – either homopolysaccharide (e.g. dextran and levan) or hetropolysaccharide (e.g. alginate). These capsular polysaccharides are composed of different sugars (glucose, galactose, mannose, rhamnose etc.) and sugar acids (glucuronic acid, mannuronic acid etc.). Polypeptide capsules consisting of D- and L-amino acids are found in some bacteria like Bacillus anthracis and Xanthomonas sp.
Plate 3. Electronmicrograph of an unidentified rod shaped bacteria showing distinct capsule surrounding the cells.
Table 1 . Nature of capsular substances produced by different bacteria
Bacterial species Capsular substances
Streptococcus mutans Homopolysaccharide (polymer of glucose)
Xanthomonas campestris Heteropolysaccharide (contain glucose, mannose, glucuronic acid)
Streptococcus pneumoniae Type II Heteropolysaccharide (contain glucose, rhamnose, glucuronic acid)
Pseudomonas aeruginosa Polysaccharide (contain mannuronic acid, glucuronic acid) Bacillus anthracis Polypeptide (polymer of glutamic acid)
Bacillus megaterium Polypeptide (polymer of glutamic acid)
The S-layer: Many Gram-nergative and Gram-positive bacteria, as well a many Archaea possess a regularly structured layer called an S-layer attached to the outermost portion of their cell wall. It is composed of protein or glycoprotein and performs a number of possible functions like protection of bacteria from harmful enzymes, changes in pH, and from the predatory bacterium Bdellovibrio. The S-layer also enables the bacterium to adhere to host cells and environmental surfaces and protect them from phagocytosis.
Figure 4. Diagrammatic representation of the internal structures of the bacterial cell
Cell wall
All bacterial cells with the exception of mycoplasmas are enveloped by a semi-rigid cell wall.
In bacteria (eubacteria) it is composed of peptodoglycan, while in Archaea (archaebacteria) it is composed of protein or pseudomurein, which is chemically distinct from peptidoglycan.
Peptidoglycan, also called murein, mucopeptide or mucopolysaccharide is a vast polymer consisting of interlocking chains of identical peptidoglycan monomers and forms a sac-like three dimensional covering around the cell. The primary chemical structures of peptidoglycans have been established. It consists of a glycan part that constitutes the backbone of the peptidoglycan and is made of long chains of two amino sugars, N- acetylglucosamine (NAG) and its lactyl ether, N-acetylmuramic acid (NAM) joined together by β-1,4-glycosidic bond. These glycan chains remain parallel to each other. Tetrapeptides of L-alanine-D-glutamic acid-L-lysine (or diaminopimelic acid)-D-alanine are linked through the carboxyl group by amide linkage of muramic acid residues of the glycan chains. The terminal D-alanine residues of one tetrapeptide are directly cross-linked to the e-amino group of lysine or diaminopimelic acid on a neighboring tetrapeptide, or a peptide bridge links them. In Staphylococcus aureus peptidoglycan, a glycine pentapeptide bridge links the two adjacent peptide structures. The extent of direct or peptide-bridge cross-linking varies from one peptidoglycan to another and provide tremendous strength to the cell wall. The staphylococcal peptidoglycan is highly cross-linked, whereas that of Eschericia coli is comparatively much less.
Most bacteria can be placed into either Gram-positive or Gram-negative groups based on their color after specific staining procedures commonly called Gram staining named after Hans Christian Gram (1835-1938) who developed this staining procedure. The procedure involves treating a heat-fixed bacterial smear with crystal violet followed by a dilute iodine solution as mordant. This is followed by treatment of cells with ethyl alcohol. This treatment results two different responses depending on the nature of the cell wall. Some bacteria retain the initial violet colour of the crystal violet and appear purple when observed under the microscope and are commonly called Gram-positive (example Bacillus subtilis, Streptococcus pyogenes, Staphylococcus aureus). The other group of bacteria, which failed to retain the violet colour become colourless and pick up the counterstain of safranin. They appear pink when observed through microscope and are called Gram-negative (Escherichia coli, Pseudomonas aeruginosa, Haemophilus influenzae). The steps of Gram staining procedure and the appearance of cells at each step are shown in Figure 6. In addition, some Gram-positive bacteria resist decolorization of the initial dye carbol fuchsin with an acid- alcohol mixture during the acid-fast stain procedure. They appear red when observed through the microscope. Common acid-fast bacteria include Mycobacterium tuberculosis and Mycobacterium leprae.
Table 2. Steps of Gram staining procedure and the appearance of cells at each step Appearance of cells under microscope Step
Gram-positive bacteria Gram-negative bacteria
Cells smeared and fixed on slide
Cells colourless Cells colourless
Primary stain: Crystal violet
Cells stain violet Cells stain violet
Mordant: Gram’s iodine
Cells remain violet Cells remain violet Differentiation: Ethyl alcohol/
acetone Cells remain violet Cells become colourless
Counterstain: Safranin
Cells remain violet Cells stain red
The Gram-positive cell wall: In electron micrographs of ultrathin sections of gram-positive bacteria the cell walls appear as a broad dense covering of 20-80 nm thick and consisting of numerous interconnecting layers of peptidoglycan . The peptidoglycan represents 60 to 90%
of the gram-positive cell wall. Teichoic acids, the polyphosphate polymers are associated with the cell wall of Gram-positive bacteria as secondary wall component and represent nearly 10-50% of the wall material. Teichoic acids are of two different types: i) polymers of glycerol phosphate called glycerol teichoic acid (found in Bacillus subtilis) and ii) polymers of ribitol phosphate called ribitol teichoic acid (found in Staphylococcus aureus).
Teichouronic acid consisting of long chains of alternating glucouronic acid and N- acetylgalactosamine linked to each other by 1-3 glycosidic bond is found in Bacillus licheniformis. Some teichoic acids have lipids attached to them and are called lipoteichoic acids.
The peptidoglycan in the Gram-positive cell wall prevents osmotic lysis. The techoic acids probably help make the cell wall stronger. The surface proteins in the bacterial peptidoglycan, depending on the strain and species, function as enzymes, help the bacteria to adhere and colonize.
Figure 5. Diagrammatic representation of Gram-positive and Gram-negative cell wall of bacteria
The Gram-negative cell wall: In Gram-negative bacteria cell wall appears multilayered. It consists of (i) a thin layer (2-3 nm thick) of peptidoglycan which represents only 10-20% of the cell wall component and (ii) an outer membrane, a lipid bilayer of about 7 nm thick. The space between the cytoplasmic membrane and the outer membrane is known as periplasmic space, which remain filled with gelatinous material called periplasm. It contains a variety of enzymes for nutrient breakdown as well as binding proteins to facilitate the transfer of nutrients across the cytoplasmic membrane. The outer membrane is composed of phospholipids, lipoproteins, lipopolysaccharides (LPS), and proteins. Phospholipids are located mainly in the inner layer of the outer membrane, as are the lipoproteins that connect the outer membrane to the peptidoglycan. The lipopolysaccharides, located in the outer layer of the outer membrane, consist of a lipid portion called lipid A embedded in the membrane and a polysaccharide portion extending outward from the bacterial surface called and is differentiated into R core region (core polysaccharide) and O side chain (O-antigen). The LPS portion of the outer membrane is also known as endotoxin. The outer membrane also contains many cross membrane channels that allow diffusion of small molecules. These pores are composed of proteins called porins.
The outer membrane because of its semipermeable nature helps retain certain enzymes and prevents entry of some toxic substances, e.g., penicillin G and lysozyme etc. The LPS portion of the outer membrane, when released, functions as a harmful endotoxin.
The acid - fast cell wall: In addition to peptidoglycan, the acid-fast cell wall of Mycobacterium contains a large amount of glycolipids, especially mycolic acids that make up approximately 60% of the acid-fast cell wall. The mycolic acids along with other glycolipids prevent the entry of variety of substances causing the organisms to be more resistant to chemical agents and lysosomal components of phagocytes that normally kill the bacteria.
The archaebacterial cell wall: The archaebacteria are characteristically different from eubacteria. Like bacteria most archaebacteria do contain cell wall with the exception of
ical nature. In
e, also called a cell membrane or plasma membrane lies internal to e cell wall and encloses the cytoplasm of the bacterium. It is about 7 nm thick and appears
chaebacteria are structurally very similar.
However, the chemical composition of eubacterial cell membranes is significantly different
arious biochemicals into and out of the cells. In simple diffusion, solute molecules move Thermoplasma species. Cell wall-containing archaebacteria can stain either Gram-positive or Gram-negative. Unlike bacteria all archaebacteria, however lack murein.
Gram-positive archaea have rigid cell wall sacculi similar to the ultrastructure of Gram- positive bacteria and are composed of polymers of diverse chem
Methanobacterium and Methanothermus the cell wall contain pseudomurein or pseudopeptidoglycan which contain N-acetyl talosaminuronic acid instead of muramic acid and N-acetyl glucosamin which are linked to each other by a β- 1,3 glycosidic bond and alternate to form the cell wall backbone. In addition, cell walls of many archaebacteria are compsed of polysaccharides of different types. Archaea that stain Gram-negative (e.g.
Methanolobus, Sulfolobus, Thermoproteus, Desulfurococcus and Pyrodictum) possess only proteinaceous or glycoproteinaceous cell envelopes (S layers). They lack the outer membrane and complex lipopolysaccharide found in Gram-negative bacteria.
Cytoplasmic Membrane The cytoplasmic membran th
as 2 dark bands separated by a light band when observed under electron microscope.
Chemically it is composed of phospholipid (20-30%) and protein (60-70%). Structurally the cytoplasmic membrane conforms to the fluid mosaic model. According to this model, the phospholipids form a bilayer in which the hydrophilic or polar ends of the molecules form the outermost and innermost surface of the membrane while the non-polar or hydrophobic ends form the center of the membrane. The protein molecules are partly or wholly embedded in the lipid layer and are differentiated as integral proteins and peripheral proteins. Integral or intrinsic proteins remain embedded into the lipid bilayer, while the peripheral or extrinsic proteins remain adhered to the hydrophilic layer of the membrane. With the exception of the mycoplasmas, the prokaryotic cell membranes lack sterols. Sterols, such as cholesterol are found only in eukaryotic cell membrane.
he cytoplasmic membranes of eubacteria and ar T
from the arcahebacteria and this is one of the most important characteristics that distinguish eubacteria from archaebacteria. The eubacterial cytoplasmic membranes contain straight chain fatty acids that are linked to glycerol by ester linkage, while the archaebacteria have membranes composed of branched hydrocarbon chains attached to glycerol by ether linkages.
The cell membrane is a selectively permeable membrane and controls the movement of v
from higher concentration to lower concentration across the membrane. Osmosis is the process of diffusion in which water molecules move through the selectively permeable membrane from a region of low concentration of solute to a region where the concentration of solute molecules is high. The pressure that encourages the movement of water is called osmotic pressure. Cytoplasmic membranes also help in the transport of substances across the membrane by transport (carrier) proteins. In addition, it acts as the site of energy production through the membrane bound electron transport system, peptidoglycan synthesis, phospholipid synthesis, division of the nucleoid and formation of endospores.
The cytoplasm and cytoplasmic structures
The cellular components that are often found in bacterial cytoplasm include the nucleoid or e nuclear material, plasmids, the ribosomes, endospores and various inclusion bodies, and organelles used for photosynthesis.
s to everything enclosed by the cytoplasmic ed of 80% water and contains nucleic acids
ntracellular enzymes, bacteria also
arge subunit (50S subunit) is composed of
ain in a supercoiled form. Super coiling of th
Cytoplasm: In bacteria, the cytoplasm refer embrane. The cytoplasm of bacteria is compos m
(DNA and RNA), enzymes, amino acids, carbohydrates, lipids, inorganic ions, and many low molecular weight compounds. Some bacteria produce cytoplasmic inclusion bodies of various types that carry out specialized cellular functions.
The cytoplasm contains a large number of enzymes and is the main site of bacterial metabolism, which includes both catabolic, and anabolic reactions. In catabolic reactions molecules are broken down in order to obtain building block molecules for more complex molecules and macromolecules, while anabolic reactions are concerned with synthesis of
ther molecules and macromolecules. Apart from these i o
produce and secrete extracellular enzymes, which are transported across the membrane and hydrolyze macromolecules into smaller molecules
Ribosomes: Ribosomes are nucleoproteinaceous particles and acts as the site of protein synthesis. It contains 60% ribosomal RNA (rRNA) and 40% protein. Complete ribosome is a 70S particle ("S" refers to Svedberg unit.) and is about 25nm in diameter. It is composed of two subunits with densities of 50S and 30S. The small subunit (30S subunit) contains about
1 proteins and a 16S rRNA molecule, while the l 2
approximately 34 proteins and one molecule each of a 23S and 5S rRNA. These two subunits combine during protein synthesis to form a functional 70S ribosome.
The function of ribosomes is to carry out protein synthesis. During protein synthesis, the message in mRNA molecules is translated to amino acid sequence in a protein. This process is known as translation and involves, apart from ribosomes, amino acid-carrying tRNAs, a number of enzymes and energy in form of ATP.
The Nucleoid: The bacterial cell, as a prokaryotic structure, lacks a distinct membrane-bound nucleus. Electron micrograph of ultra-thin sections of cells reveals that the nuclear material occupies a position near the centre of the cell as a light fibrilar area and is referred to as the nucleoid. The nucleoid consists of a long, single, circular chromosome composed of double
tranded deoxyribonucleic acid or DNA that rem s
this DNA macromolecule is accomplished by a group of enzymes called DNA topoisomerases to fit it into the bacterium. In addition to topoisimerases, a number of proteins involved in DNA replication (DNA polymerase) and transcription (RNA polymerase) are also found to be associated with bacterial chromosome. Although bacteria generally lack the basic histone proteins, histone-like proteins have been reported in some arcahebacteria and eubacteria.
The bacterial nucleoid does not divide by mitosis and since it contains a single chromosome (haploid) and only reproduce asexually, there is also no meiosis in bacteria. The chromosome is generally around 1000 µm long and frequently contains as many as 3500 genes. Cells of Escherichia coli, which are 2-3 µm in length, have a chromosome approximately 1400 µm
ng.
lo
Since the nucleoid contains most of the genetic information of the bacterium it determines the synthesis of proteins and enzymes of an organism and there by regulates the overall biochemical reactions of the bacterial cell.
Plasmids: In addition to the nucleoid, many bacteria often contain one or more, small,
de an advantage under certain environmental onditions like resistance to antibiotics, heavy metals, degradation of toxic compounds etc.
cted bacterial genera like, Bacillus, Clostridium, Desulfotomaculum, porosarcina, Sporolactobacillus, and Thermoactinomyces produce endospores. They are
a) Terminal spore without swelling of mother cell. (b) Central spore without swelling of mother cell. (c) Terminal spherical spore, mother cell distended. (d) Sub-terminal spherical spore mother cell distended. (e)Lateral spore, mother cell distended.Endospores may be e central, ubterminal or terminal. Each bacterial cell forms a single endospore and the mother cell in circular, nonchromosomal DNA molecules called plasmids. Plasmids contain only a limited amount of genetic information and are not essential for normal bacterial growth and survivability. They can, however, provi
c
Moreover, plasmids impart pathogenicity in many bacteria where the production of toxins are encoded by plasmids.
Endospore: Endospores are specialized resting bodies formed within the body of a small group of bacteria and remain in a dormant or metabolically inert form. But they are highly resistant to the lethal effect of heat, drying, freezing, deleterious chemicals and radiation.
Members of some sele S
unusually dehydrated and appear highly refracticle under microscope. They could also be stained selectively with specific dye called malachite green.
Figure 6 . Drawings showing the location of endospores in spore-forming bacteria.
(
spherical, ellipsoidal or cylindrical in shape and in the cell there position may b s
which the spore is produced is called a sporangium. A mature spore may have a diameter same as, or greater than that of the vegetative cell. The latter causes a bulging of the cell, if it is central it is called clostridium and if terminal a plectridium. As a rule each bacterial species has its own characteristic size, shape and position of the spore but this is subject to variation under different environmental conditions.
ndospores are formed under conditions of nutrient limitation, especially the lack of carbon and itrogen sources. The process of spore formation is called sporogenesis. The process of
orogenesis is depicted in Figure . It gives rise to a single endospore within a single cell, hich is novel in structure and composition from the mother cell that produces it. During ndospore formatio a unique compound called dipicolinic acid is synthesized and Ca++ is ccumulated in the spore. The dipicolinic acid form chelate with Ca++ to form calcium ipicolonate, the concentration of this compound has been implicated with the heat resistance f the bacterial endospore.
he mature endospore as revealed by the electron microscope consists of a very thick nvelope, which is distinguished as spore coat, cortex and spore wall. Spore coat is the utermost layer of the spore and consists of two layers: the outer coat and the inner coat. In me species the spores are surrounded by a exosporium outside the coat. Beneath the spore oat is the cortex, which is followed by a very thin spore wall surrounding the interior or core f the spore. The core is equivalent to cytoplasm of the vegetative cell and contains a ucleoid, some ribosomes, RNA molecules, and enzymes.
he endospores are liberated upon autolysis of the vegetative cell. The mature spores are
metabolically i cals. The
esistance of endospores to physical and chemical agents is due to a variety of factors which include: (i) low water content or dehydrated condition of the spores, (ii) abundance of
Figure 7 . Drawings showing the ultrastucture of bacterial endospore
E
n sp w e a d o
T e o so c o n T
nert but exhibit high degree of resistance to heat, radiation and chemi r
calcium-dipicolinate that stabilizes and protect the endospore's DNA, (iii) presence of specialized DNA-binding proteins saturate the endospore's DNA and protect it from heat,
drying, chemicals, and radiation, and (iv) DNA repair enzymes contained within the endospore are able to repair damaged DNA during germination.
In suitable nutrient media most of the spores germinate to give rise vegetative cells. Though some bacterial spores germinate spontaneously in a favourable medium, others remain dormant unless heat or chemical substances first activate them. During germination water is
apidly imbibed and the physio-biochemical activities of the spores increases rapidly and the
r granules that store sulfur. These
ystals of magnetite or other
anoxygenic photosynthesis. Their
ems that are continuous with the cytoplasmic
y different nutritional requirements that remain dissolved in water.
hydrogen, oxygen, nitrogen, sulfur, phosphorus, potassium, r
vegetative cell eventually bursts out of the spore coat.
Inclusion bodies: Bacteria when grown under different environmental conditions are found to synthesize and accumulate variety of chemical substances as insoluble deposits in their cytoplasm commonly called inclusion bodies.
Some bacteria produce inorganic inclusion bodies in their cytoplasm including polyphosphate or metachromatic granules that store phosphate, and sulfu
may serve as energy reserve for the bacteria.
Many bacteria also accumulate either polyhydroxybutyrate granules (Ralstonia eutropha) or glycogen granules as carbon and energy reserve. Polyhydroxybutyrate granules are accumulated when the bacteria are grown under nutrient limiting condition in presence of excess of carbon sources.
Some motile aquatic bacteria (e.g. Aquaspirillum magnetotacticum) have been found to possess magnetosomes. Magnetosomes are membrane-bound cr
iron-containing substances that function as tiny magnets.
Autotrophic bacteria (Thiobacillus, Nitrosomonas etc.) that reduce CO2 in order to produce carbohydrates, possess carboxysomes containing ribulose bis posphate carboxylase, an enzyme used for CO2 fixation.
The green bacteria (e.g. Chlorobium) carry out
photosynthetic system is located in ellipoidal vesicles called chlorosomes that are independent of the cytoplasmic membrane. The purple bacteria (e.g. Rhodopseudomonas, Rhodospirillum) also carry out anoxygenic photosynthesis but their photosynthetic system is located in spherical or lamellar membrane syst
membrane.
Nutrition and Growth of Bacteria Nutrition
Bacteria in order to grow require drawing from its environment all the necessary substances for synthesis of their cellular components and generation of energy. The chemicals and elements of this environment that are utilized for bacterial growth are referred to as nutrients.
Different bacteria have ver
Like all other living systems, water is the main component for growth of the bacteria. The nutritional requirements of a bacterium are evident from the elemental composition of the bacterial cells, which are broadly categorized as macroelements or major elements and microelements or trace elements.
Macroelements or major elements are required by the bacteria in high concentration. Ten macroelements such as carbon,
calcium, magnesium and iron are found in the form of water, inorganic ions, small molecules,
denum etc.
bacterial cell for its growth in nature or in the laboratory ust have a supply of carbon source and a source of energy. Carbon, the main constituent of presents nearly 50% of bacterial cell dry weight. The carbon requirements of
herefore, based on the type of carbon and energy sources for growth all bacteria are
hotoheterotrophs or Photoorganotrophs: They use light as the energy source and an organic
urce of energy and CO2 as the principal carbon source. These acteria obtain their energy by the oxidation of H2, NH3, NO2, H2S. A few bacteria and many
3 and 1% of cell dry weight spectively. Phosphorus is supplied to bacterial cells as inorganic phosphates and serves as
t their supply of sulfur from inorganic and macromolecules, which serve either a structural or functional role in the cells.
Microelements or trace elements are metal ions required by cells in small amounts. These trace elements usually act as cofactors for essential enzymatic reactions in the cell. Trace elements in bacterial nutrition include manganese, zinc, copper, chlorine, sodium, cobalt, nickel, molyb
Carbon and energy sources: Every m
the cell re
bacteria are fulfilled either by CO2 or organic carbon. Organisms that use CO2 as a sole source of carbon for growth are called autotrophs and that use organic carbon are known as heterotrophs. The organic carbon sources include simple sugars, amino acids, proteins, complex carbohydrates and lipids. As source of energy many bacteria use the radiant energy (light) and are called phototrophs, while organisms that use (oxidize) an organic form of carbon are called heterotrophs or chemo(hetero)trophs. Organisms that oxidize inorganic compounds are called lithotrophs (Thiobacillus, Nitrobacter).
T
distinguished into four major nutritional categories.
Photoautotrophs: They use light as the energy source and CO2 as the sole source of carbon.
Some purple and green sulfur bacteria belong to this category.
P
compound as the principal carbon source. This category includes some purple and green sulfur bacteria.
Chemoautotrophs or Lithotrophs (Lithoautotrophs): They use inorganic compounds like H2, NH3, NO2, H2S as the so
b
archaea belong to this category.
Chemoheterotrophs or Heterotrophs: They derive their energy as well as carbon source from the metabolism of a single organic compound. Most bacteria belong to this group.
Nitrogen: Nitrogen represents nearly 14% of the dry weight of bacterial cells. Majority of bacteria thrive on inorganic nitrogen compounds like ammonium salts, nitrates and nitrites, while others obtain nitrogen from organic nitrogenous compounds like amino acids. A few bacteria are able to use atmospheric nitrogen.
Phosphorus and sulfur: Phosphorus and sulfur represent re
essential component of nucleotides, nucleic acids, phospholipids etc. Sulfur is required for synthesis of some amino acids. Some bacteria ge
sulfates, some from organic sulfur compounds and some also utilize elemental sulfur.
In addition to these non-metallic elements, bacteria also require metal ions, such as K+, Mg++, Ca++, Fe++ as cofactors for many enzymes and are essential for normal growth. Other metal ions such as Zn++, Cu++, Mn++, Mo++, Ni++, Co++ etc. are often required in trace amount and serve as the cofactor for many enzymes.
in organic ompounds, which they cannot synthesize from available nutrients. Such compounds are
ins: needed as coenzymes and functional roups of certain enzymes.
These ompounds are to be added to the culture media, which are used for their growth. The growth
ctors are not metabolized directly as sources of carbon or energy, rather they are ssimilated by cells to fulfill their specific role in metabolism.
acterial Growth
rowth is reflected by an increase size while in
a flask and is incubated under proper environmental conditions, e population will pass through distinct phases of growth. The changes in number of cells or Growth Factors: Apart from the requirement of carbon and energy source, macro- and microelements, many of the bacteria may require small amounts of certa
c
required in small amounts and are called growth factors. The need for growth factors in bacteria varies from species to species and are broadly categorized into (i) purines and pyrimidines: required for synthesis of nucleic acids (DNA and RNA); (ii) amino acids:
required for the synthesis of proteins, and (iii) vitam g
Some bacteria like E. coli do not require any growth factors. They can synthesize all essential purines, pyrimidines, amino acids and vitamins, starting with their carbon source, as part of their own intermediary metabolism. Certain other bacteria (for example Lactobacillus) require purines, pyrimidines, vitamins and several amino acids in order to grow.
c fa a B
The term growth as it is applied in biology refers to an irreversible increase in cellular mass due to synthesis of all its essential constituents and is accompanied by an increase in cell numbers. In multicellular organisms, g
unicellular organisms like bacteria growth refers to an increase in the number of cells in a population. Therefore, growth of bacteria is usually measured by measuring the increase in cell numbers, increase in dry weight of cell mass or by monitoring the uptake and synthesis of cellular components like DNA, RNA, protein etc. and is expressed in the form of a growth curve.
When a population of viable bacterial cells (inoculum) is introduced into a fresh liquid growth medium contained in
th
the biomass are plotted against time to obtain a sigmoid growth curve. The typical growth curve as illustrated in Figure 14 shows four distinct phases: the lag phase, the exponential or logarithmic phase, the stationary phase and the death or decline phase. In between each of these phases there is a transitional phase.
hydrate
cellu e in
ent s f the growth environment.
ial cells undergo rapid nential increase of the population and the cell mass. The generation time of bacteria i.e. the time required for the Figure 8 . Typical bacterial growth curve showing phases of growth
The first phase, the lag phase encompasses several hours and represents the time between the addition of inoculation and the beginning of the second phase, the exponential phase. During this period the cells do not divide immediately, but the individual cells grow in size and synthesize all essential organic constituents like protein, nucleic acid and carbo
through active metabolism. In other words, during this phase the cell division lags behind lar metabolism. However, the bacterial cells start dividing showing a slow increas population at the end of this phase. This phase is also considered as the phase of adjustm during which bacterial cells adapt themselves with the new physical and chemical condition o
During the next phase, the logarithmic phase (log phase), the bacter cell division at a constant rate resulting in logarithmic or expo
population to double can be determined from this phase, which is characteristic for individual bacterial species under optimum growth conditions. For example, in suitable medium and temperature the generation time of Eschericia coli is 20-30 min., while for slow growing organisms like Mycobacterium tuberculosis it is 344 – 461 min. During this phase, the cells are most nearly uniform in terms of their chemical composition, metabolic activity and physiological characteristics. Cells from logarithmic phase exhibit their highest metabolic and physiological activities.
During the third phase, the stationary phase, the rate of cell division decreases and the older cells begin to die. This might be due to accumulation of toxic metabolic products and exhaustion of essential nutrients from the growth medium. As a result the population appears constant with no net increase in cell number. During this phase bacteria used to accumulate a number of secondary metabolites like antibiotics.
The stationary phase is followed by the death phase during which death rate exceeds the rowth rate. The number of viable cells decreases exponentially and there may be very few iable cells at the end of this phase. The increased death rate is due to further accumulation of
hibitory metabolic products and almost total deletion of nutrients.
hysical and Environmental Requirements for Bacterial Growth
ffect of Oxygen: Oxygen is a universal component of cells. Depending on the requirement
switch between aerobic nd anaerobic types of metabolism. Under anaerobic conditions (without O2) they grow by hey switch to aerobic spiration; iv) Aerotolerant anaerobes are bacteria with an exclusively anaerobic
ffect of pH: The pH, or hydrogen ion concentration, [H+], of the environment is an
required for growth, acteria are classsified (Figure 10) as: i) mesophiles, organisms growing best within a g
v in P
Bacteria in order to grow require a wide range of physical and environmental conditions. The optimum condition for the bacterial growth varies depending on the nature of the organism and the habitat in which it grow.
E
of oxygen bacteria are classified as: i) Obligate aerobes, which require O2 for growth. They use O2 as a final electron acceptor in aerobic respiration; ii) Obligate anaerobes do not use O2
as a nutrient. In fact, O2 is a toxic substance, which either kills or inhibits their growth; iii) Facultative anaerobes (or facultative aerobes) are organisms that can
a
fermentation or anaerobic respiration, but in the presence of O2 t re
(fermentative) type of metabolism but they are insensitive to the presence of O2. They live by fermentation alone whether or not O2 is present in their environment.
E
important requirement for growth of the bacteria. The range of pH over which an organism grows is differentiated as the minimum pH, below which the organism cannot grow, the maximum pH, above which the organism cannot grow, and the optimum pH, at which the organism grows best. Microorganisms, which grow at an optimum pH well below neutrality (7.0) are called acidophiles (Thiobacillus, Acidithiobacillus, Sulfolobus species). Those which grow best at neutral pH are called neutrophiles (Staphylococcus aureus) and those that grow best under alkaline conditions are called alkaliphiles.
Effect of Temperature: A particular microorganism usually exhibits a range of temperature over which it can grow. Considering the total span of temperature
b
temperature range of 25-40°C; ii) thermophiles, organisms with an optimum temperature between about 45°C and 70°C; iii) extreme thermophiles or hyperthermophiles, organisms with an optimum temperature of 80°C or higher and a maximum temperature as high as 115°C; and iv) psychrophiles, the cold-loving organisms that are able to grow at 0°C or lower but have an optimum temperature of 15-20°C.
Figure 9 . Classification of bacteria based on temperature requirement
Figure 9 . Classification of bacteria based on temperature requirement
Water Availability: Water is the solvent in which the molecules of life are dissolved,
availability of water is, therefore, a critical factor that affects the growth of all bacteria. The availability of water for a cell depends upon its presence in the atmosphere (relative
idity) or its presence in solution or a substance (water activity). The water activity (A of pure H2O is 1.0 (100% water). Water activity is affected by the presence of solutes such as salts or sugars that are dissolved in the water. The higher the solute concentration of substance, the lower is the water activity and vice-versa. Microorganisms live over a range of
from 1.0 to 0.7.
Sodium chloride: The only common solute in nature that occurs over a wide concentration range is salt [NaCl], and some microorganisms are named based on their growth response to salt. Microorganisms that require some NaCl for growth are halophiles. Mild halophiles
and the
hum w)
a Aw
require les that require
15-30% NaCl for growth are found among the archaea. Bacteria that are able to grow at
t generation.
with the inward growth or invagination of the cytoplasmic membrane and formation of new 1-6% salt, moderate halophiles require 6-15% salt; extreme halophi
moderate salt concentrations, even though they grow best in the absence of NaCl, are called halotolerant.
Bacterial Reproduction
In case of bacteria, growth is equivalent to reproduction as the growth of individual cell is accomplished by the reproduction of the entire organisms in which the genetic information is transmitted to the nex
Binary fission: Binary fission is the most common mode of reproduction in bacteria. In binary fission a cell divides to produce two equal-sized daughter cells (Figure 10). The process of cell division requires the doubling of bacterial chromosome prior to cell division so that each daughter cell receives a complete genome. The process of binary fission starts
cell wall material, which ultimately separates the two daughter cells. This is commonly called septa formation. Formation of septa or cross wall physically separates the two complete
acterial chromosomes in two daughter cells. This ensures the equal distribution of complete As result of cross wall formation two equal sized cells are formed.
igure 10 . Diagram showing bacterial multiplication by binary fission.(a) Mother cell, ) Cell enlargement, (d) Initiation of septum formation, (d) Transverse septum formation and qual distribution of cell components, (e) Separation of daughter cells
udding: Budding is a process in which small protrusion expands outward from a mother cell rming a daughter cell. The daughter cell increases in size until it breaks off from the mother ell. In budding the cell wall extends from one point instead of growing evenly throughout e cell, which allows polar division. During the process a copy of the chromosome from the other cell must pass into the daughter cell before the bud breaks off. This type of division is und in Culobacter, Rhodomicrobium etc.
b
genome between the cells.
Figure 10 . Diagram showing bacterial multiplication by binary fission.
F (b e B fo c th m fo
Fragmentation: Filamentous bacteria such as Nocardia reproduce by fragmentation of the laments into rod-shaped or coccoid cells. Each of these bacillary or coccoid cells can give se to a new organism.
ristic es or simply conidia eveloped singly or in chains from the tips of the filamentous hyphae (e.g. Streptomyces).
icroorganisms are widely distributed in our environment and are involved in every aspects
ay a major role in the production of food and fermented beverages including dairy roducts, alcoholic beverages, fish and meat products along with a variety of fermented plant products.
airy Products: Milk from cow, buffalo, sheep, goat and horse are used as raw materials for fi
ri
Formation of spores: Bacteria belonging to the group Actinomycetes form characte branched filamentous hyphae similar to eukaryotic fungi. These filamentous bacteria reproduce by the formation of asexual spores called conidiospor
d
During the formation of spores the hyphal tip undergoes septation by crosswall formation to form chain of spores. In some members like Streptosporangium, the spores are enclosed within specialized sac called sporangium and the spores are called sporangiospores. The spores are produced in large numbers and each of them on germination gives rise to a new organism.
Economic Importance of Bacteria M
of human life. They have long been exploited in manufacturing of industrial products and medicines, in bioremediation of polluted ecosystems and also in increasing soil fertility.
Biotechnological advancement has led to exploring the versatile nature of microorganisms towards the benefit of mankind. Some of the major applications of microorganisms are enumerated below:
Food and Fermented beverages Bacteria pl
p
D
making of cheese, yogurt, butter, sour cream, etc. Traditionally, they are produced by lactic acid bacteria, like Lactococcus, Lactobacillus, Leuconostoc, Streptococcus, etc. Table 3 illustrates examples of some fermented dairy products and the producer organisms.
Table 3. Examples of fermented dairy products and microorganisms involved in their production.
Products Raw ingredients Fermenting microorganisms Acidophilus milk Milk Lactobacillus acidophilus Soft Cheese (unripened)
Cottage Milk curd ctococcus lactis, Leuconostoc citrovorum Soft Cheese (Ripened, 1-5 months)
Camembert Milk curd ctococcus lactis, Lactococcus cremoris Semi-soft Cheese (Ripened, 1-12 months)
d, 3-12 mo
Cheddar Milk curd Lactococcus lactis, Lactobacillus casei, Lactococcus cremoris, Strptococcus durans Yogurt Milk,
milk solids
Streptococcus thermophilus, Lactobacillus delbrueckii ssp. bulgaricus
La
La
Roquefort Milk curd Lactococcus lactis, Lactococcus cremoris Hard Cheese (Ripene ths)
roducts st ts with inoculation of pasteurized milk with a
specif ium. Ferm enerates l roteins and
forms flavor and aroma compounds. Cheese manufacture involves formation of solid curd id whey and ripening of curd. Hundreds of varieties of cheese
are available throughout the worl used,
ripening period and metho cessing. So airy product
usually prepared from whole milk using str illus. The
production of butter involves ripening of pasteurized cream with Lactococcus lactis and ostoc citrovorum fo 48 hours pri
Alcoholic beverages: Around the world different
and distilled spirits are red thro tation of fruits juices, ydrolysed grain and root starch. Though bacteria such as Zymomonas species are involved
the production of these beverages, yeasts like Kluveromyces and Saccharomyces cerevisiae are primarily used. These organisms ferment sugar and produces ethanol along
randy (from wine and cider).
Manufacturing of these fermented p ar
ic bacter entation g actic acid, which modifies milk p from milk, removal of the liqu
d depending upon the type of
ds of pro milk and microorganism
ur milk or yogurt is also a major d ains of Streptococcus and Lactobac
Leucon r 24 to or to churning.
types of alcoholic beverages like wine, beer ugh microbial fermen
manufactu h
in
with compounds imparting aroma and flavor and carbondioxide. Most of the beverages are aged to modify the flavor and increase alcohol concentration. Wine is produced mainly from grape vines because it contains high level of fermenting sugar, pleasant flavor and inhibits growth of spoilage organisms. The immense range of wine depends on variety of grapes, cultivation conditions, fermentation processes and post fermentation treatments. Cider is the alcoholic beverage prepared from apple juice and contains 2-8% ethanol. Beer is the non- distilled alcoholic beverage containing 3-8% ethanol and made from partially germinated cereal grains termed as malt. Distillation of the intermediate products during production of wine, beer, etc. results in formation of whisky (from beer) or b
Fermented food: The fermented meat products like sausages, pepperoni, salami, etc. are produced by the activity of bacteria Pediococcus cerevisiae, Lactobacillus plantarum and Staphylococcus carnosus. Meat is allowed to ripen after adding salt and bacteria for several days and a change in texture, color and flavor is noted. Fermented fish products like sauces and paste are used as flavoring agents and produced by fermentative activity of Staphylococcus carnosus and S. piscifermentans. Several locally available fruits, vegetables, cereal grains, legumes, oilseeds are used for preparation of fermented food. Fermented soya beans are a major dietary source in South-east Asia. Soy sauces, mainly used as condiments and flavoring agents is prepared using Pediococcus acidophilus. Sauerkraut or fermented cabbage, is prepared by salting shredded cabbage and fermenting with natural microflora like Lactobacillus plantarum, L. brevis, etc. along with Leuconostoc mesenteroides for acidification.
Health-care products
Antibiotics are the most important group of compounds produced by industrially important bacteria and they have been used for the last 60 years in improving human health. The other major health-care products are vaccines, vitamins, amino acids, proteins, etc. Antibiotics are secondary metabolites produced by variety of microorganisms and are used as antimicrobial chemotherapeutic agents. The first antibiotic, penicillin, was discovered by A. Flemming in 1928 from the fungus Penicillium notatum. Later Waksman discovered streptomycin (1946) from filamentous bacteria Streptomyces griseus. Since then a wide variety of antibiotics have been discovered from bacteria, particularly the filamentous group called actinomycetes. Some of the common antibiotics and their producer organisms are shown in table 4. An ideal or broad spectrum antibiotic should be active against Gram-positive as well as Gram-negative bacteria. Commercial production of antibiotic involves growth of the suitable organism in a rge submerged tank called biofermentor for 7-8 days and desired compound is recovered
by filtration, adsorption, precipitation and purification.
la
from the culture filtrate
Table 4 . Some common antibiotics and their producer organisms
Antibiotic Bacteria Bacitracin Bacillus licheniformis
Polymyxin B Paenibacillus polymyxa Streptomycin Streptomyces griseus
Neomycin S. fradiae
Chloramphenicol S. venezuelae
Tetracyclin S. rimosus
Erythromy Nystatin
Amphotericin B S. nodosus
cin S. erythreus
S. noursei
Vita cobalamine or Vita commercially by fermentation of Propionibacterium freudenreichii an d as a by product of antib tation from Strepto Another vitamin, pantothenic acid is produced from immobilized cells of coli. Precursor of vitamin C or ascorbic acid is also produced by Gluconobacter.
Vaccines: of bacteria or its inactivated forms are used for preparation of vaccines (Table 5). They are the most important tool for fighting of infec al diseases. Comme tion requires growth of the desired organism in large quantities followed by subsequent treatment and testing for safe use. Bacterial protein toxins can also serve as vaccines following their inactivation with formaldehyde or heat to form n metabolic by products of bacteria produced by recombinant DNA tech being used as vac
Table 5. Examples of some bacterial vaccines applied for medical use mins: Cyano min B12 is produced
d P. shermani. It is also recovere myces olivaceous.
iotic fermen
Escherichia
Live attenuated (weakly virulent) strains
tious bacteri rcial produc
toxoids. In additio
nology are also cines.
Vaccine Disease Live attenuated vaccine
Bacillus anthracis Anthrax
Salmonella typhi Typhoid
Inactivated vaccine
Neisseria meningitides Meningites
Vibrio cholerae Cholera
Toxoids
Clostridium tetani Tetanus
Corynebacterium diphtheria Diphtheria
Proteins: R utic protein
and peptide like insulin, human growth hormone, etc. Therapeutic proteins have also been developed for treatment of cancer and viral diseases, cardiovascular diseases, neurological disorders, e richia coli, Bacillus subtilis and Pseudomonas aeruginosa
have been used for p b ugs.
Industrial Chemicals and Fuel
Bacteria have been tion of a huge variety of industrial agents including enzymes, organic a l polymers and polysacch ofuel production also involves mi
Microbial Enzymes: ployed for commercial production of a large number of en lication in laundry, paper, textile and leather
dustries and various pharmaceutical preparations. These enzymes include amylase, rotease, glucose isomerase, etc. (Table 6). Commercially produced bacterial enzymes are
ecombinant DNA technology has allowed production of many therape
tc. Apart from Esche
reparation of these cost effective recom inant dr
employed for genera
cids, microbia arides. Bi
crobial fermentation.
Many bacterial strains are em zymes which find app
in p
nowadays replacing chemical catalysts because they are easy to produce, cheaper and environment friendly.
Table 6. Examples of some bacterial enzymes and their industrial application
Enzyme Source Applications
Amylase Bacillus subtilis Starch processing, baking, textile manufacture, etc.
B. licheniformis
Pullulanase Klebsiella aerogenes Starch processing Glucose isomerase B. coagulans Production of syrup
Streptomyces galbus
Protease B. subtilis Biological detergents, meat tenderization and cheese manufacture
Alkaline protease B. licheniformis Laundry detergents
Amino acids: Microbial production of important amino acids is preferred because they are biologically active.
roteins Several
acteria like Corynebacterium, Arthrobacter, Brevibacterium, etc. produce large amount of a ds in the cult ium from where it nd later purified. Commercially produced amino acids include lysine, glutamic acid, m
Organic acids: Acetic acid, citric acid, lactic a
microbial fermentation and used in the food industry as an acidulant and flavoring agent.
T lso used i etergen pharmaceutical processes.
Vinegar is a condiment % acetic acid m maple syrup, molasses, honey, cereals, root starch, etc. or alcoholic beverages like wine, cider, spirit alcohol, etc.
Acetic acid fermentati erobic condition using strains of Acetobacter and Gluconobacter. Lactic good solvent a
Besides, it is also a good preservative. In commercial plants, lactic acid bacteria like Leuconostoc delbrukii, L. bulgaricus, L. pentosus, Streptococcus lactis, etc. are grown in a
edium of semi-refined sugars and the lactic acid thus produced is recovered in crystalline
ithin the next few years has led to the search for alternative energy sources. Apart from geothermal, nuclear, wind, water and solar energy, They can be used as a supplement for animal as well as vegetable , as food additives to improve taste and for pharmaceutical preparations.
p b
mino aci ure med is isolated a
ethionine, etc.
cid, gluconic acid, etc. are produced from hey are a n electroplating, d t industry and
containing 4 . It is produced fro on occurs under a
acid is nd finds application in polymer industry.
m
form as calcium lactate by addition of CaCO3.
Microbial polymers: Bacteria synthesizes a host of biopolymers mostly polysaccharides like xanthan, dextran, curdlan, etc. Xanthan gum produced by Xanthomonas campestris is used in preparation of food like sauces, syrup, etc., in paint and textile industries, making of explosives, deodorants, etc. Dextran, a polymer of α-glucose, is obtained from Acetobacter, Klebsiella and Leuconostoc and finds major application in pharmaceutical industry. Curdlan,
es faecalis is also used in pharmaceutical prep
from Alcaligen arations. Bacteria like
Alcaligenes, Pseudomonas, Bacillus, Azotobacter, etc. under suitable growth conditions produce large amount of polymers inside the cell as reserve substances. Chemically they are poly-β-hydroxybutyrate (PHB). It is biodegradable in nature, offering a desirable alternative to synthetic plastics. The bioplastics obtained from microbial sources are nowadays being used in medical and packaging industries.
Biodetergents: Microbial products like glycolipids from Pseudomonas aeruginosa, Rhodococcus erythropolis and Bacillus subtilis have been found to possess characteristics similar to detergents and are, therefore, used for preparation of environment-friendly biodetergents and biosurfactants. They are used for emulsification, wetting, dispersion, solubilization, etc.
Biofuel: The depletion of fossil fuel supplies w
