MICROBIOLOGY PPT LECTURE NOTES | Karya Tulis Ilmiah

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Bacterial

Genetics

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68%

Deinococcus

radiodurans

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PLASMIDS

PLASMIDS Plasmids are extrachromosomal genetic elements capable of autono

Plasmids are extrachromosomal genetic elements capable of autono

mous replication. An

mous replication. An episome

episome

is a plasmid that can integrate into the bacterial chromo

some

Classification of Plasmids



Transfer properties



Conjugative plasmids



Nonconjugative plasmids



Phenotypic effects



Fertility plasmid (F factor)



Bacteriocinogenic plasmids

.

(8)

Insertion sequences (IS)-

Insertion sequences are transposable genetic

elements that carry no known genes except those that are required for transposition.

•

a. Nomenclaturea. Nomenclature - Insertion sequences are given the designation IS followed by a number. e.g. IS1 - Insertion sequences are given the designation IS followed by a number. e.g. IS1

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b. Structureb. Structure Insertion sequences are small stretches of DNA that have at their ends repeated sequences, Insertion sequences are small stretches of DNA that have at their ends repeated sequences, which are involved in transposition. In between the terminal repeated sequences there are genes involved i

which are involved in transposition. In between the terminal repeated sequences there are genes involved i

n transposition and sequences that can control the expression of the genes but no other nonessential gene

s are present.

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c. Importancec. Importance

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i) Mutation - The introduction of an insertion sequence into a bacterial gene will result in the inactivation of ti) Mutation - The introduction of an insertion sequence into a bacterial gene will result in the inactivation of t he gene.

he gene.

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ii) Plasmid insertion into chromosomes - The sites at which plasmids insert into the bacterial chromosome aii) Plasmid insertion into chromosomes - The sites at which plasmids insert into the bacterial chromosome a re at or near insertion sequence in the chromosome.

re at or near insertion sequence in the chromosome.

•

iii) Phase Variation - The flagellar antigens are one of the main antigens to which the immune response is diiii) Phase Variation - The flagellar antigens are one of the main antigens to which the immune response is di rected in our attempt to fight off a bacterial infection. In Salmonella there are two genes which code for two

rected in our attempt to fight off a bacterial infection. In Salmonella there are two genes which code for two

antigenically different flagellar antigens. The expression of these genes is regulated by an insertion sequen

ces. In one orientation one of the genes is active while in the other orientation the other flagellar gene is acti

ve. Thus, Salmonella can change their flagella in response to the immune systems' attack. Phase variation i

s not unique to Salmonella flagellar antigens. It is also seen with other bacterial surface antigens. Also the

mechanism of phase variation may differ in different species of bacteria (e.g. Neisseria; transformation).

(9)

Transposons (Tn) -

Transposons are transposable genetic elements th

at carry one or more other genes in addition to those which are essential for tran

sposition.

•

Nomenclature

- Transposons are given the designation Tn followed by a n

umber.

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Structure - The structure of a transposon is similar to that of an insertion s

Structure

- The structure of a transposon is similar to that of an insertion s

equence. The extra genes are located between the terminal repeated sequen

ces. In some instances (composite transposons) the terminal repeated seque

nces are actually insertion sequences.

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Importance

- Many antibiotic resistance genes are located on transposons.

Since transposons can jump from one DNA molecule to another, these anti

biotic resistance transposons are a major factor in the development of plas

mids which can confer multiple drug resistance on a bacterium harboring s

uch a plasmid. These multiple drug resistance plasmids have become a maj

or medical problem because the indiscriminate use of antibiotics have prov

ided a selective advantage for bacteria harboring these plasmids

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Mechanism of bacterial

variation

•

Gene mutation

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Gene transfer and recombinati

on

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Transformation

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Conjugation

•

Transduction

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Types of

mutation

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Base substitution

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Frame shefit

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What can cause mutation?

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Chemicals:

nitrous acid; alkylating agents

5-bromouracil

benzpyrene

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Radiation: X-rays and Ultraviolet

light

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Bacterial

mutation

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Mutation rate

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Transformation

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Transformation is gene transfer resulting f

rom the uptake by a recipient cell of nake

d

DNA from a donor cell. Certain bacteria

(

e.g.

Bacillus, Haemophilus, Neisseria, Pn

eumococcus)

can take up DNA from the e

nvironment and the DNA that is taken up

can be incorporated into the

recipient's c

hromosome.

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Conjugation

•

Transfer of DNA from a donor to a recipie

nt by direct physical contact between th

e

cells. In bacteria there are two mating t

ypes a donor (male) and a recipient (fem

ale) and the direction

of transfer of gene

tic material is one way; DNA is transferre

d from a donor to a recipient.

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Physiological States of F

Factor

•

Autonomous (F

+

)

–

Characteristics of F

++

x F

-

-crosses

crosses

•

F

--

becomes F

++

while F

++

remains F

++

•

Low transfer of donor

chromosomal genes

(23)

Physiological

States

of F

Factor

•

Integrated (Hfr)

–

Characteristics of

Hfr x F

--

crosses

•

F

--

rarely becomes

Hfr while Hfr rem

ains Hfr

•

High transfer of c

ertain donor chro

mosomal genes

(24)

Physiological States of F

Factor

•

Autonomous with

donor genes (F’)

–

Characteristics of

F’ x F

--

crosses

•

F

--

becomes F’

while F’ remains

F’

•

High transfer of

donor genes on

F’ and low

transfer of other

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Mechanism of F

+

x F

-

Crosses

•

DNA transfer

–

Origin of

transfer

–

Rolling circle

replication

• Pair formation

– Conjugation

bridge

F

+

F

-

F

₊

F

(26)

Mechanism of Hfr x F

-

Crosses

•

DNA transfer

–

Origin of transfer

–

Rolling circle

replication

•

Homologous

recombination

• Pair formation

– Conjugation

bridge

Hfr

F

-

Hfr

F

(27)

-Mechanism of F’ x F

Mechanism of F’ x F

-

Crosses

•

DNA transfer

–

Origin of

transfer

–

Rolling circle

replication

• Pair formation

– Conjugation

bridge

F’

F

-

F’

(28)

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Transduction:

•

Transduction is defined as the transfer o

f genetic information between cells thro

ugh the mediation of a virus (phage) par

ticle. It therefore does not require cell to

cell contact and is DNase resistant.

(30)

Generalized Transduction

•

Generalized transduction is transduction

in which potentially any

bacterial gene fr

om the donor can be transferred to the r

ecipient.

(31)

The mechanism of

generalized

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Generalized

transduction

1.

It is relatively easy.

2.

It is rather efficient (10

-3-3

per recipient

with P22HT, 10

-6-6

with P22 or P1), using

the correct phage.

3.

It moves only a small part of the

chromosome which allows you to change

part of a strain's genotype without

affecting the rest of the chromosome.

4.

The high frequency of transfer and the

small region transferred allows

fine-structure mapping

(36)

Specialized transduction

•

is transduction in which

only

certain donor

genes

can be transferred to th

e recipient.

•

Different phages may transfer different genes but a

n

individual phage can only transfer certain genes

•

Specialized transduction is mediated by lysogenic

or temperate phage and the genes that get transfer

red will depend on where the prophage has

insert

ed in the chromosome.

(37)

The mechanism of specialized

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1.

Very efficient transfer of a small region--can be us

eful for fine-structure mapping

2.

Excellent source of DNA for the chromosomal regi

on carried by the phage, since every phage carrie

s the same DNA.

3.

Can often be used to select for deletions of some

of the chromosomal genes carried on the phage.

4.

Merodiploids generated using specialized phage

can be quite useful in complementation analyses

.

(39)

Lysogenic conversion

•

The prophage DNA as a gene r

ecombined with chromosome

of host cell.

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Protoplast Fusion

•

Fusion of two protoplasts treated with ly

sozyme and penicillin.

(41)

Application of Bacterial

Variation

•

U

se in medical clinic: Diagnosis,

Treatment, Prophylaxis.