Detection
of other microbial species and the host
environment by Salmonella.
We
are studying metabolic and environmental
inputs to virulence gene regulation in Gram-negative
pathogens, primarily Salmonella.
The two regulators studied most intensely,
sdiA and sirA, are located
adjacent to each other on the chromosome.
SdiA. SdiA is a transcription factor
of the LuxR family that is used to detect bacterial pheromones (N-acylhomoserine
lactones, or AHLs). SdiA detects only the pheromones
made by other species of bacteria. In the
figure below wild-type Salmonella
is using SdiA to detect pheromones synthesized
by Yersinia enterocolitica. This was done by fusing one of the genes that SdiA regulates to luciferase. The Salmonella and Yersinia are simply streaked perpendicular to each other on an LB plate. The second picture used a fusion to lacZ instead of luciferase. LacZ activity is indicated by the blue color. This experiment used motility agar so that the Salmonella and Yersinia could swim to each other in the plate. In both pictures note that only the wild-type Salmonella can detect the Yersinia. The sdiA mutant is "blind". Also, Salmonella can only detect the wild-type Yersinia. The yenI mutant of Yersinia cannot make pheromones. Using
a genetic screen we have determined that
in Salmonella, SdiA activates a horizontal acquisition named srgE for which the function is unknown, and a small cluster of genes known as the rck operon. This operon is located on the Salmonella virulence plasmid and confers adhesion to epithelial cells and resistance to killing by mammalian complement systems. We are currently using a technique called RIVET to determine when and where Salmonella detects other microbial species in host animals. Our most recent discovery is that Salmonella appears to detect Aeromonas hydrophila in turtles (see papers below).
 
Papers on SdiA:
Brian M. M. Ahmer, Jeroen van Reeuwijk, Cynthia D. Timmers, Peter J. Valentine, and Fred Heffron. 1998. Salmonella typhimurium encodes an SdiA homolog, a putative quorum sensor of the LuxR family, that regulates genes on the virulence plasmid. Journal of Bacteriology 180: 1185-1193.
Bindhu Michael, Jenee N. Smith, Simon Swift, Fred Heffron, and Brian M. M. Ahmer. 2001. SdiA of Salmonella enterica is a LuxR homolog that detects mixed microbial communities. Journal of Bacteriology 183: 5733-5742.
Jenee N. Smith and Brian M. M. Ahmer. 2003. Detection of other microbial species by Salmonella: Expression of the SdiA regulon. Journal of Bacteriology 185: 1357-1366.
Brian M. M. Ahmer. 2004. Cell to cell signaling in Escherichia coli and Salmonella enterica. Molecular Microbiology, 52: 933-945.
Amber Lindsay and Brian M. M. Ahmer. 2005. The effect of sdiA on biosensors of N-acylhomoserine lactones. Journal of Bacteriology 187: 5054-5058.
Brian M. M. Ahmer, Jenee N. Smith, Jessica L. Dyszel, and Amber Lindsay. 2007. Methods in cell-to-cell signaling in Salmonella. In Schatten, Heide; Eisenstark, Abe (Eds.), Salmonella: Methods and Protocols (pp. 307-322). Humana Press, New Jersey.
Jenee N. Smith, Jessica L. Dyszel, Jitesh A. Soares, Craig D. Ellermeier, Craig Altier, Sara D. Lawhon, L. Garry Adams, Vjollca Konjufca, Roy Curtiss III, James M. Slauch, and Brian M. M. Ahmer. 2008. SdiA, an N-Acylhomoserine Lactone Receptor, Becomes Active during the Transit of Salmonella enterica through the Gastrointestinal Tract of Turtles. PLoS ONE 3(7): e2826.
SirA. The gene downstream of sdiA is present throughout the gamma-proteobacteria and regulates virulence genes in all of these organisms. However, unlike SdiA, we do not know the signal for SirA. The signal that is activating virulence gene expression in so many pathogens is a very significant topic. So far we know that SirA is phosphorylated by a sensor kinase named BarA and we know many of the genes that are activated by SirA. Interestingly, two of the genes activated by SirA are regulatory RNAs named csrB and csrC. They in turn inhibit the activity of an RNA binding protein named CsrA. When CsrA is not inhibited, it regulates carbon storage genes and many of the same virulence genes that SirA regulates. The hilA and hilC genes in the diagram below are regulators that control one of the Type III secretion systems that is a primary virulence factor of Salmonella. Thus, SirA is activating transcription of virulence genes and controlling the translation of those same genes via CsrA. This regulatory network is an active area of investigation in our lab.
Papers on SirA:
Brian M. M. Ahmer, Jeroen van Reeuwijk, Patricia R. Watson, Tim S. Wallis, and Fred Heffron. 1999. Salmonella SirA is a global regulator of genes mediating enteropathogenesis. Molecular Microbiology 31: 971-982.
Brian M. M. Ahmer, Mimi Tran, and Fred Heffron. 1999. The virulence plasmid of Salmonella typhimurium is self-transmissable. Journal of Bacteriology 181: 1364-1368.
Robert I. Goodier and Brian M. M. Ahmer. 2001. SirA orthologs affect both motility and virulence. Journal of Bacteriology 183: 2249-2258.
Max Teplitski, Robert I. Goodier, and Brian M. M. Ahmer. 2003. Pathways leading from BarA/SirA to motility and virulence gene expression in Salmonella. Journal of Bacteriology, 185: 7257-7265.
Max Teplitski and Brian M. M. Ahmer. 2005. The control of secondary metabolism, motility, and virulence by the two-component regulatory system BarA/SirA of Salmonella and other g-proteobacteria, pp 107-132, In B. M. Pruess (ed.), “Complex regulatory networks in enteric bacteria,” Research Signpost, Trivandrum, India.
Max Teplitski, Robert I. Goodier, and Brian M. M. Ahmer. 2006. Catabolite repression of the SirA regulatory cascade in Salmonella. International Journal of Medical Microbiology, 296: 449-466.
Max Teplitski, Ali Al-Agely, and Brian M. M. Ahmer. 2006. Contribution of the SirA regulon to biofilm formation in Salmonella enterica serovar Typhimurium. Microbiology, 152: 3411-3423.
Yakhya Dieye, Jessica L. Dyszel, Rebin Kader, and Brian M. M. Ahmer. 2007. Systematic analysis of the regulation of type three secreted effectors in Salmonella enterica serovar Typhimurium. BMC Microbiology, 7: 3.
Current Lab Members (Spring 2009):
Jay Soares, Ph.D., Jessica Dyszel, Ph.D., Darren Lucas, Mat Vross, Matt Swearingen (not shown), Mohamed Ali, Olga Zemska.
Ahmer Lab alumni:
Rob Goodier (postdoc 1999-2001), currently at Q-One Biotech, Scotland
Yakhya Dieye (postdoc 2002-2005), currently at Ondek, Perth Australia
Max Teplitski (postdoc 2002-2005), currently an Assistant Professor at University of Florida
Amber Lindsay (M.S. 2006), currently a Senior Microbiology Technician at Battelle, Columbus OH
Jenee Smith (Ph.D. 2007), currently a researcher at Battelle, Columbus OH
Jessica Dyszel (Ph.D. 2009), hasn't quite left the Ahmer lab yet...
Undergraduate Research Opportunities:
Dr. Ahmer's lab has opportunities for undergraduate research. If you have received an A in Micro 581 and have a cumulative GPA of at least 3.0, send an email.
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