Rasesh Y. Parikh

Extracellular Synthesis of Crystalline Silver Nanoparticles and Molecular Evidence of Silver Resistance from Morganella sp : Towards Understanding Biochemical Synthesis Mechanism

There has been significant progress in the biological synthesis of nanomaterials. However, the molecular mechanism of synthesis of such bio‐nanomaterials remains largely unknown. Here, we report the extracellular synthesis of crystalline silver nanoparticles (AgNPs) by using Morganella sp., and show molecular evidence of silver resistance by elucidating the synthesis mechanism. The AgNPs were 20±5 nm in diameter and were highly stable at room temperature. The kinetics of AgNPs formation was investigated. Detectable particles were formed after an hour of reaction, and their production remained exponential up to 18 h, and saturated at 24 h. Morganella sp. was found to be highly resistant to silver cations and was able to grow in the presence of more than 0.5 mM AgNO3. Three gene homologues viz. silE, silP and silS were identified in silver‐resistant Morganella sp. The homologue of silE from Morganella sp. showed 99 % nucleotide sequence similarity with the previously reported gene, silE, which encodes a periplasmic silver‐binding protein. The homologues of silP and silS were also highly similar to previously reported sequences. Similar activity was totally absent in closely related Escherichia coli; this suggests that a unique mechanism of extracellular AgNPs synthesis is associated with silver‐resistant Morganella sp. The molecular mechanism of silver resistance and its gene products might have a key role to play in the overall synthesis process of AgNPs by Morganella sp. An understanding of such biochemical mechanisms at the molecular level might help in developing an ecologically friendly and cost‐effective protocol for microbial AgNPs synthesis.

Bacteria-mediated precursor-dependent biosynthesis of superparamagnetic iron oxide and iron sulfide nanoparticles.

The bacterium Actinobacter sp. has been shown to be capable of extracellularly synthesizing iron based magnetic nanoparticles, namely maghemite (gamma-Fe2O3) and greigite (Fe3S4) under ambient conditions depending on the nature of precursors used. More precisely, the bacterium synthesized maghemite when reacted with ferric chloride and iron sulfide when exposed to the aqueous solution of ferric chloride-ferrous sulfate. Challenging the bacterium with different metal ions resulted in induction of different proteins, which bring about the specific biochemical transformations in each case leading to the observed products. Maghemite and iron sulfide nanoparticles show superparamagnetic characteristics as expected. Compared to the earlier reports of magnetite and greigite synthesis by magnetotactic bacteria and iron reducing bacteria, which take place strictly under anaerobic conditions, the present procedure offers significant advancement since the reaction occurs under aerobic condition. Moreover, reaction end products can be tuned by the choice of precursors used.

Tracking the influence of long-term chromium pollution on soil bacterial community structures by comparative analyses of 16S rRNA gene phylotypes.

Bacterial community structures of highly chromium-polluted industrial landfill sites (G1 and G2) and a nearby control site (G3) were assessed using cultivation-dependent and cultivation-independent analyses. Sequencing of 16S rRNA genes discerned a total of 141 distinct operational taxonomic units (OTUs). Twelve different bacterial phyla were represented amongst 35, 34 and 72 different bacterial genera retrieved from sites G1, G2 and G3, respectively. The bacterial community of site G1 consisted of Firmicutes (52.75%), Gammaproteobacteria (18%), Actinobacteria (14.5%), Bacteriodetes (9.5%) and Deinococcus-Thermus (5.25%) and that of site G2 consisted of Firmicutes (31.25%), Alphaproteobacteria (7%), Betaproteobacteria (8%), Gammaproteobacteria (19%), Deltaproteobacteria (9.5%), Epsilonproteobacteria (3%), Actinobacteria (13%), Bacteriodetes (7.75%) and Deinococcus-Thermus (1.5%). The bacterial community of site G3 consisted of Firmicutes (6.25%), Alphaproteobacteria (7.5%), Betaproteobacteria (17.25%), Gammaproteobacteria (29.75%), Deltaproteobacteria (7.5%), Epsilonproteobacteria (4%), Actinobacteria (9.5%), Bacteriodetes (11.25%), Gemmatimonadetes (2.5%), Deinococcus-Thermus (1.8%), Chloroflexi (1.5%) and Planctomycetes (1.2%). The phyla of Gemmatimonadetes, Chloroflexi and Planctomycetes were not detected in sites G1 and G2; likewise, Alpha, Beta, Delta and Epsilon subdivisions of Proteobacteria were not recovered from site G1. These findings reveal that long-term chromium-induced perturbation results in community shifts towards a dominance of Firmicutes from Proteobacteria in the soil environment.

Genus-Wide Physicochemical Evidence of Extracellular Crystalline Silver Nanoparticles Biosynthesis by Morganella spp

This study was performed to determine whether extracellular silver nanoparticles (AgNPs) production is a genus-wide phenotype associated with all the members of genus Morganella, or only Morganella morganii RP-42 isolate is able to synthesize extracellular Ag nanoparticles. To undertake this study, all the available Morganella isolates were exposed to Ag+ ions, and the obtained nanoproducts were thoroughly analyzed using physico-chemical characterization tools such as transmission electron microscopy (TEM), UV-visible spectrophotometry (UV-vis), and X-ray diffraction (XRD) analysis. It was identified that extracellular biosynthesis of crystalline silver nanoparticles is a unique biochemical character of all the members of genus Morganella, which was found independent of environmental changes. Significantly, the inability of other closely related members of the family Enterobacteriaceae towards AgNPs synthesis strongly suggests that AgNPs synthesis in the presence of Ag+ ions is a phenotypic character that is uniquely associated with genus Morganella.

Midgut Microbial Community of Culex quinquefasciatus Mosquito Populations from India

The mosquito Culex quinquefasciatus is a ubiquitous species that serves as a major vector for west nile virus and lymphatic filariasis. Ingestion of bloodmeal by females triggers a series of physiological processes in the midgut and also exposes them to infection by these pathogens. The bacteria normally harbored in the midgut are known to influence physiology and can also alter the response to various pathogens. The midgut bacteria in female Cx. quinquefasciatus mosquitoes collected over a large geographical area from India was studied. Examination of 16S ribosomal DNA amplicons from culturable microflora revealed the presence of 83 bacterial species belonging to 31 bacterial genera. All of these species belong to three phyla i.e. Proteobacteria, Firmicutes and Actinobacteria. Phylum Proteobacteria was the most dominant phylum (37 species), followed by Firmicutes (33 species) and Actinobacteria (13 species). Phylum Proteobacteria, was dominated by members of γ-proteobacteria class. The genus Staphylococcus was the largest genus represented by 11 species whereas Enterobacter was the most prevalent genus and recovered from all the field stations except Leh. Highest bacterial prevalence was observed from Bhuj (22 species) followed by Nagrota (18 species), Masimpur (18 species) and Hathigarh (16 species). Whereas, least species were observed from Leh (8 species). It has been observed that individual mosquito harbor extremely diverse gut bacteria and have very small overlap bacterial taxa in their gut. This variation in midgut microbiota may be one of the factors responsible for variation in disease transmission rates or vector competence within mosquito population. The present data strongly encourage further investigations to verify the potential role of the detected bacteria in mosquito for the transmission of lymphatic filariasis and west nile virus. To the best of our knowledge this is the first study on midgut microbiota of wild Cx. quinquefasciatus from over a large geographical area.