The School of Molecular and Cellular Biology at the University of Illinois at Urbana-Champaign

Salmonella cause 1.4 million cases of gastroenteritis and enteric fever per year in the US and lead all other foodborne bacterial pathogens as a cause of death. The long-term objectives of our research are to understand the molecular mechanisms by which Salmonella circumvents the host immune system to cause disease. Salmonella typhimurium provides an ideal model system to study molecular pathogenesis. The genetics of this organism are well defined and allow the simple manipulation and characterization of mutations that affect virulence. In addition, there is an excellent animal model of infection to study the effects of bacterial mutations on pathogenesis. It has been our goal to take full advantage of the genetic power of the system in our pathogenic studies.

The most serious Salmonella disease results from extraintestinal infection and bacteremia. Children, the elderly, and immunocompromised individuals are particularly susceptible to more serious Salmonella infection. The hallmark of these extraintestinal infections is the ability of Salmonella to survive in macrophages, which normally kill bacteria by producing a variety of antimicrobials, including superoxide and other reactive oxygen and nitrogen species. Evidence that phagocyte-produced superoxide is important in Salmonella infection is clear; mice and humans who are genetically defective in superoxide production are significantly more susceptible to infection. However, the molecular mechanism by which external superoxide kills or inhibits Salmonella is not known.

S. typhimurium resistance to the oxidative burst of phagocytes requires periplasmic Copper/Zinc co-factored superoxide dismutase (SodC), which detoxifies superoxide. Our strain of S. typhimurium produces two periplasmic superoxide dismutases.  SodCII is chromosomally encoded, whereas SodCI is encoded on the fully functional bacteriophage Gifsy-2.  We have shown that only SodCI contributes to virulence in the animal.  In contrast, SodCII is not required during infection, even in the absence of SodCI.  Both proteins are made during infection and are enzymatically indistinguishable, suggesting that some physical difference between the two proteins allows SodCI, but not SodCII, to contribute to virulence.  From our work and that of others we know that SodCI is dimeric, protease resistant, and tethered within the periplasm by some non-covalent ionic interaction.  SodCII is monomeric, protease sensitive, and released normally from the periplasm by several different methods.  Our current working model proposes that macrophages deliver to the Salmonella-containing phagosome a variety of antimicrobial molecules, including antimicrobial peptides, proteases, and superoxide.  The antimicrobial peptides, at least transiently, disrupt the outer membrane of the bacterium.  Periplasmic proteins, like SodCII, are released and/or phagocytic proteases gain access to the periplasm.  SodCII is degraded under these conditions.  In contrast, SodCI is tethered within the periplasm and is inherently protease resistant.  Therefore, it remains to detoxify the phagocytic superoxide.  We are currently testing aspects of this model and performing a detailed structure/function analysis of SodCI.

The fundamental question of how phagocytic superoxide damages or kills Salmonella remains.  We have generated extensive genetic data showing that phagocytic superoxide damages an extracytoplasmic target; cytoplasmic targets including DNA seem to be irrelevant.  However, we do not know the nature of the target(s).  Several projects are focused on determining the targets of superoxide and the overall mechanism of bacterial inhibition.  Our recent studies have identified a series of genes that show genetic interactions with SodCI.  Characterization of these genes and their respective pathways will help us understand the bacterial physiology of external superoxide stress.

Our second area of study is the regulation of the Type III Secretion System (T3SS) encoded on Salmonella Pathogenicity Island 1 (SPI1). This complex of over 30 proteins is transcriptionally induced in response to a variety of environmental signals such that the machinery is produced at the appropriate time when the bacteria are in the intestine of the host. Using this machine, Salmonella injects bacterial proteins, termed effectors, into the epithelial cells leading to actin rearrangement and invasion of the bacterium. Using careful genetic analyses, we have modeled the complex circuitry responsible for integrating these various environmental signals. The master SPI1 regulatory gene hilA is controlled directly by three AraC-like regulators: HilD, HilC and RtsA.  HilC and HilD are encoded in the SPI1 locus, while RtsA is encoded elsewhere in the chromosome in an operon with RtsB, which negatively regulates expression of flhDC and therefore the entire flagellar regulon.  HilC, HilD and RtsA are each capable of activating expression of hilC, hilD and rtsA, creating a complex feed forward regulatory loop that regulates hilA expression.

We now understand that the global regulatory systems feed into SPI1 through HilD.  Our current work is focused on understanding how several global regulators affect HilD function at the molecular level.  The data to date suggests that this regulation is post-translational, probably acting at the level of HilD protein. This is the key to understanding SPI1 regulation.