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Dipshikha Chakravortty
Ph.D., University of Poona, Pune
Phone: +91 80 22932842/ +91 80 22932986
E-mail: dipa@mcbl.iisc.ernet.in
URL : http://mcbl.iisc.ernet.in/Welcome%20to%20MCBL/Faculty/Dipshikha/Dipshikha.htm

We are studying the factors involved in pathogenesis of Salmonella typhimurium.  We are specifically working on elucidating the mechanism of antimicrobial peptide resitance.
The ability of a pathogen to establish itself successfully in the host requires an efficient networking of the genes, which are involved in disarming the host defense mechanism. Intracellular pathogen like Salmonella typhimurium has evolved excellent strategies to evade its killing by the host. Keeping in mind a large set of cells which Salmonella can infect, leads one to speculate the intricate regulation of distinct set of genes in Salmonella needed for survival in different host cell types. In macrophages, where phagocyte oxidative and nitrosative burst peaks, Salmonella induces set of genes to combat the oxidative or nitrosative stresses. Further, Salmonella can replicate within the macrophages by avoiding fusion with lysosome. In dendritic cells, it is still a mystery, as to why the Salmonella population is static. The microarray data revealed a set of genes which are upregulated in the macrophage population, one among them being yejF. Further bioinformatic analysis showed that the products of the yej genes have significant similarity with sap (sensitivity to antimicrobial peptides) genes.  Elegant experiments by Groisman and his colleagues have shown that phoP/Q and sap mutants which showed increased sensitivity to antimicrobial peptides (AMPs), have reduced virulence, suggesting that resistance to host AMPs plays an important role in Salmonella pathogenesis. Thereafter, the role of sap in pathogenesis was shown in Haemophilus influenzae and Erwinia chrysanthemi. As the route of infection of Salmonella is oro-fecal, the first cells it comes in contact with are the gastrointestinal epithelial cells. Intestinal epithelial cells, especially Paneth cells, produce several AMPs. AMPs are widely distributed in phylogenetically diverse animals, plants and bacterial species. Most of these peptides are cationic in nature that allows interactions with the bacterial cytoplasmic membrane which usually comprises negatively charged phospholipids. AMPs exhibit broad antimicrobial activity against Gram-negative and Gram-positive bacteria, fungi, and enveloped viruses. They also function as chemotactic agents, cytotoxins and opsonins.

Resistance to antimicrobial peptides, is one of the important strategic plans executed by Salmonella to successfully establish in the host. Bioinformatic analysis revealed that products of the putative genes encoded by yejABEF (yej operon) operon possess significant similarity with those of the sapABCDF operon (sap operon). The sap operon in Salmonella encodes an ABC transporter and has been shown to confer resistance to the AMPs. Further, yej operon was reported to be up-regulated inside the host cells almost ten folds. In this study, we have addressed the question whether yej operon joins the set of genes which confer resistance against AMPs like sap, phoP, pmr etc.  We have deleted the yejF gene which encodes the ATPase component of the putative ABC transporter encoded by yej operon. The yejF strain showed increased sensitivity to antimicrobial peptides like protamine, melittin, polymyxin B and human defensin HBD-2 and was compromised in its capacity to proliferate inside human epithelial cells and macrophages but not inside mouse macrophages. In murine typhoid model the yejF strain displayed decreased virulence as evidenced by the increased survival of the mice infected with the yejF strain when compared to those infected with the wild-type strain. Taken together, these findings suggest that yej operon is plays a pivotal role in Salmonella pathogenesis.

Selected Publication

Sandeepa M. Eswarappa , Kiran Kumar Panguluri, Micheal Hensel and Dipshikha Chakravortty (2008)   yejF, a virulence gene in Salmonella enterica Typhimurium is involved in counteracting anti-microbial peptides. Microbiology-SGM: 154: 666-678

Sandeepa M. Eswarappa , Nirmalya Basu, Omana joy and  Dipshikha Chakravortty  (2008) Folimycin (concanamycinA) inhibits LPS-induced nitric oxide production and reduces surface localization of TLR4 in murine macrophages. Innate Immunity 14(1):13-24.

Jonathan Jantsch, Dipshikha Chakravortty, Nadine Turza, Alexander T. et al.(2008),Hypoxia and Hypoxia Inducible Factor-1Modulate Lipopolysaccharide-Induced Dendritic Cell Activation and Function, J. Immunol., 180(7)

Negi VD, Singhamahapatra S, Chakravortty D.(2007) Salmonella enterica serovar Typhimurium strain lacking pmrG-HM-D provides excellent protection against salmonellosis in murine typhoid model. Vaccine ;25(29):5315-23.