1.5 Recent trends in nanobiotechnology
1.5.6 Antibacterial activities of silver nanoparticles synthesized from plant extracts
The anti-microbial history of silver dates back to the Phoenicians who recognized silver and used as a natural biocidal agent. Silver is even known to prevent HIV binding to host cells (Elechiguerra et al., 2005). Silver itself has a toxic effect on microorganisms and chemically synthesized AgNPs are reported to damage nucleic acids (Awasthi et al., 2013). Nanoparticles lead to an increase in reactive oxygen species (ROS) associated with DNA damage, apoptosis, and necrosis (Foldbjerg et al., 2011). Ag NPs are known to reduce mitochondrial function, increase membrane leakage in mammalian germline stem cells and increase ROS generation, deplete antioxidant reduced glutathione (GSH) content, and reduce mitochondrial function in rat liver cells (Braydich-Stolle et al. 2005).
Brahmaputra River floodplain represents a huge consortium of plant genetic resources of endemic origin and high ethnobotanical values. Considering the outputs of prevalent conventional methods of bioresource utilization, researchers have prioritized green synthesis of nanoparticles, utilization of the medicinal values of plants against the survival of pathogenic bacteria in water and soil. Green synthesized nanoparticles are capped with phytochemicals that mask the toxic effect of AgNPs, the antimicrobial activity is triggered by the large surface area of the small sized capped AgNPs available for interaction and hence more effective for biocidal activities than the larger NPs (Kvitek et al., 2008). The mechanism of antimicrobial activity may possibly involve growth inhibition by formation of free radicals that act upon the lipid membranes of bacterial cells. AgNPs may act differentially on Gram-positive and Gram-negative
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bacteria due to differences in their cell wall structure i.e. presence of peptidoglycan layer (Kim et al., 2007). Li et al (2010) speculated that antimicrobial activity of AgNPs is initiated as a leakage of membrane followed by leakage of reducing on respiratory chain dehydrogenases. Amro et al (2010) recommended that metal depletion may lead to formation of irregularly shaped pits in the outer membrane and change membrane permeability, followed by progressive release of lipopolysaccharide molecules and membrane proteins. This mechanism defies the actual concept of pit formation and disruption of cell wall, because there has been no clear evidence of either positively charged Ag+ ions or negatively charged AgNPs are responsible for deformity of cell wall and pit formation (Sondi et al., 2004). This hypothesis is further supported by Kim et al (2009) and Li et al (2010), as they suggest release of silver ions from the AgNPs that may contribute to their bactericidal properties.
Antimicrobial activities of green synthesized AgNPs from plant extracts have been broadly studied. Some of them are Argemone mexicana against Escherichia coli, Pseudomonas syringae and Aspergillus flavus (Singh et al., 2010); Cinnamon zeylanicum against Escherichia coli (Sathishkumar et al., 2009); Acalypha indica against Escherichia coli and Vibrio cholera (Krishnaraj et al., 2010); Ficus benghalensis against Escherichia coli (Saxena et al., 2012); Euphorbia hirta against Escherichia coli, Staphylococcus aureus, Bacillus cereus, Klebsiella pneumonia, Pseudomonas aeruginosa (Elumalai et al., 2010); Garcinia mangostana against Escherichia coli and Staphylococcus aureus (Veerasamy et al., 2011); Ocimum sanctum against Escherichia coli and Staphylococcus aureus (Singhal et al., 2011);
Mentha piperita against Escherichia coli and Staphylococcus aureus (Ali et al., 2011);
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Polyalthia longifolia against Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus (Kaviya et al., 2011); Moringa oleifera against Staphylococcus aureus, Candida tropicalis, Candida krusei and Klebsiella pneumoniae (Prasad et al., 2011); Nicotiana tobaccum against Pseudomonas aeruginosa and Escherichia coli (Prasad et al., 2011); Eucalyptus chapmaniana against Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Proteus vulgaris, Staphylococcus aureus and Candida albicans (Sulaiman et al., 2013); Citrus limon against Fusarium oxysporum and Alternaria brassicicola (Vankar et al., 2012); Mimusops elengi against Klebsiella pneumoniae, Micrococcus luteus and Staphylococcus aureus (Prakash et al., 2013);
Artemisia nilagirica, Staphylococcus aureus, Bacillus subtilis, Escherichia coli, and Proteus subtilis (Vijayakumar et al., 2013); Alternanthera sessilis against Staphylococcus aureus and Escherichia coli (Niraimathi et al., 2013); Salicornia brachiate against Staphylococcus aureus, Staphylococcus aureus, Bacillus subtilis and Escherichia coli (Seralathan et al., 2014); Annona squamosa against Bacillus cereus, Bacillus subtilis, Staphylococcus aureus, Staphylococcus typhimurium, Pseudomonas aeruginosa and Proteus vulgaris (Jagtap et al., 2013); Carthamus tinctorius against Escherichia coli, Staphylococcus aureus, Bacillus subtilis and Candida albicans (Sreekanth et al. 2011); Millingtonia hortensis against Bacillus subtilis and Klebsiella planticola (Gnanajobitha et al., 2013); Boswellia serrata against Escherichia coli, Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa, Klebsiella pneumonia and Pseudomonas putida (Kudle et al., 2013); Datura metel against Micrococcus luteus, Escherichia coli, Staphylococcus aureus and Klebsiella pneumonia (Nethradevi et al., 2012); Catharanthus roseus against Escherichia coli, Pseudomonas putida, Klebsiella pneumonia and Bacillus subtilus (Manisha et al.,
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2014); Dioscorea batatas against Staphylococcus aureus, Escherichia coli, Saccharomyces cerivisae and Candida albicans (Nagajyothi et al., 2011) and Brassica oleracea against Staphylococcus aureus and Escherichia coli (Sridhara et al., 2013).
Antimicrobial activity using green synthesized nanoparticles can be adopted as a cost effective and easy strategy for designing water filters to remove contamination of water.
In concluding remarks, it is mandatory to mention that ecological study encompasses a wide range of research perspectives. Productivity of a floodplain ecosystem is a key indicator of its health and ability to support human requirements.
Assessment of ecological parameters reflects a monitoring criterion for sustaining a healthy ecosystem. A healthy ecosystem will provide better ecosystem support to the biological components. Thus proper monitoring and evaluation of a baseline status of riverine floodplain would open up broader scope of bioresource utilization in the future.