Research

I am interested in understanding molecular and genetic mechanisms of gene regulation in bacteria including human pathogens. Currently, research in my laboratory focuses on two aspects of pathogenesis of Salmonella enterica species.

A) Molecular characterization of plasmid and integron-mediated antimicrobial resistance in Salmonella isolated from humans and animals
Multiple antimicrobial resistances in Salmonella species is a problem of both national and international importance. A number of Salmonella species carrying multi-drug resistance plasmids have been described in the last decade. The blaCMY-2 gene on these plasmids confers resistance to beta lactam antimicrobials including Ceftriaxone, a third generation cephalosporin for treating salmonellosis in children. These plasmids in Salmonella can potentially disperse blaCMY-2 gene and hence antimicrobial resistance to new Salmonella spp. as well as other enteric bacteria commonly found in the human or animal gut or even in soil environment. Thus the spread of multi-drug resistance is of major concern in treatment of Salmonellainfections. We propose to isolate and fully characterize these multi-drug resistance plasmids carrying blaCMY-2 gene or unidentified integrons from human and animal isolates of Salmonella spanning over a 10 year period. Our hypothesis is that genetic comparisons of the plasmids from Salmonella isolates 10 years apart would reveal mutations that might have accumulated making them more resistant to multiple antimicrobials and underlying mechanisms for their dispersal in nature. A total of 50 Salmonella field isolates positive for the multi-drug resistance plasmids carrying blaCMY-2 will be characterized by disc diffusion assay and MIC testing for determining the antibiotic resistance/sensitivity profile of these isolates. Our initial results show differences in antimicrobial resistance among the strains being tested. Also, replicon typing by cloning and sequencing of plasmids using primers published in the literature, has shown two types of plasmids (type B and type C) so far. Initial characterizations of one of the plasmids also show presence of class I integron. Further detailed analysis of these strains will indicate which of the drug resistance genes are carried on the integron. This study will advance our knowledge concerning the development and dissemination of multiple antimicrobial resistances among Salmonella spp. and other enteric bacteria in nature.

B) Identification and Characterization of HilD-or HilC-dependent virulence genes in Salmonella enterica serovar Typhimurium
Salmonella typhimurium is a gram negative enteric pathogen that causes gastroenteritis. Salmonella invasion into nonphagocytic epithelial cells in the small intestine is mediated by a type III secretion system (T3SS). These genes are located on a 40 kb pathogenicity island, the Salmonella pathogenicity island I (SPI-1). The SPI-1 T3SS forms a needlelike complex that enables the bacteria to deliver effector proteins from its cytosol directly into the host cell cytoplasm (Galan, J.E. 2001). HilA is a central regulator that coordinates the expression of the SPI-1 system. Expression of the hilA gene is controlled directly by three AraC-like regulators: HilC, HilD, and RtsA (Schechter, L.M., S.M. Damrauer, and C.A. Lee, 1999; Ellermeier, C.D., Ellermeier, J.R., and J.M. Slauch, 2005;). HilC and HilD are encoded on SPI-1, while RtsA is encoded elsewhere in the chromosome in an operon with RtsB. HilA, directly activates transcription of the inv/spa and prg/org promoters of SPI-1. The AraC-like protein InvF, also encoded in the inv operon, can activate transcription of other genes located within and outside of SPI-1, including sopB, encoding a secreted effector protein. We have previously shown that HilC and HilD activate transcription of the invF gene from a promoter far upstream of the HilA-dependent promoter (Akbar et al., 2003). This activation is most probably through direct binding of HilC and HilD to sequences upstream and downstream of this alternative HilA-independent promoter. Based on these findings we hypothesized that HilC and HilD have a second role in SPI-1 gene regulation that is separate from their role in coordinating expression of the SPI-1 T3SS through hilA. In this regard, we have made random transposon insertions in the Salmonella genome using Tn5 mutagenesis in three different backgrounds (ΔhilD,ΔhilAhilD andΔspi-1). Our current project is to determine if the expression of these insertions is regulated by HilD protein. We would also like to know if this expression is dependent or independent of the expression of hilA gene.

2011 research

We are interested in discerning the molecular evolution of multidrug resistance plasmids of Salmonella spp. in order to better understand the basis of genetic differences between these plasmids and the mechanism/s of their spread in nature. We are also examining the regulation of expression of Type III secretion genes in Salmonella Typhimurium and their role in bacterial virulence.

Publications

Schechter, L. M., S. Jain, S. Akbar, and C. A. Lee. The small nucleoid-binding proteins H-NS, HU, and Fis affect hilA expression in Salmonella enterica serovar Typhimurium. Infect. Immun. 71:5432-5., 2003

Akbar, S., C. P. Lostroh, L. M. Schechter, and C. A. Lee. HilD has a direct role in the activation of invasion gene expression in Salmonella typhimurium. Mol. Microbiol. 47:715-28., 2003

Akbar, S., T. A. Gaidenko, C. M. Kang, M. O'Reilly, K. M. Devine, and C. W. Price. New family of regulators in environmental signaling pathway which activates the general stress factor sBof B. subtilis. J. Bacteriol. 183:1329-1338., 2001

Akbar, S., C. M. Kang, T. Gaidenko, and C. W. Price. Modulator protein RsbR regulates environmental signaling in the general stress pathway of Bacillus subtilis. Mol. Microbiol. 24:567-578., 1997

Akbar, S., and C. W. Price. Isolation and characterization of csbB, a gene controlled by Bacillus subtilis general stress transcription factor sB. Gene 177:123-128., 1996