Infectious diarrhea is a complex syndrome caused by many viruses, parasites, fungi and bacteria such as Salmonella. Salmonellosis itself encompasses a spectrum of clinical diseases that range from enteric fever, a serious condition that kills about 600,000 people a year, to non-typhoidal zoonotic infections that affect over a billion people worldwide and more than a million Americans annually. Treatment of Salmonella and medically important Gram-negative rods is often complicated by the increasing resistance of pathogenic bacteria to antibiotics used in the clinic. In the course of human infection, Salmonella are exposed to reactive oxygen and nitrogen species produced by the NADPH phagocyte oxidase and inducible nitric oxide synthase (iNOS). Clinical and experimental observations indicate that these reactive species mediate host defense against Salmonella. However, Salmonella and a variety of clinically relevant Gram-positive and –negative bacteria can co-opt reactive oxygen and nitrogen species to foster virulence and gain resistance to various classes of antibiotics. The research in the Vázquez-Torres lab uses state-of-the-art biochemical, genetic and molecular biology approaches to understand the molecular mechanisms by which reactive species mediate resistance of macrophages against intracellular bacteria, as well as the adaptive strategies that boost antioxidant and antinitrosative defenses of pathogenic bacteria.
The survival of Salmonella within macrophages is greatly dependent on the type III secretion system encoded within the Salmonella pathogenecity island 2 (SPI2). Our investigations have found that the type III secretion system encoded in the Salmonella pathogenecity island 2 (SPI2) blocks the interactions of Salmonella vacuoles with NADPH oxidase-containing vesicles and lysosomes, thereby inhibiting the progression of the Salmonella phagosome along the degradative pathway. However, the high throughput of nitric oxide generated by macrophages in response to IFNγ inhibits the transcription of the SPI2 type III secretion system, thus allowing maturation of the Salmonella phagosome for fusion with lysosomes. The expression of SPI2 is controlled by the SsrA/SsrB two-component regulatory system. In addition to canonical phosphorylation, our investigations indicate that SsrB is also under the post-translational control of oxidation and S-nitrosylation, and that the sensing of reactive species through Cys203 in the dimerization domain of SsrB fosters Salmonella pathogenesis. Moreover, our research indicates that SsrB is also regulated via protein-protein interactions with a chaperon already present in ancestral lineages of enterobacteriaceae.
First associated with nutritional deprivation, the “stringent response” activates or represses global gene transcription through the actions of the guanine tetraphosphate (ppGpp) alarmone and the DksA regulatory protein on the RNA polymerase (RNAP). The stringent response protects phylogenetically diverse eubacteria against a variety of environmental stress conditions and promotes the virulence of medically important pathogens such as Pseudomonas aeruginosa, Vibrio cholerae, and Salmonella enterica. Our recent work indicates that reactive oxygen and nitrogen species elicit a stringent response in Salmonella. Our investigations have identified the zinc finger in the globular domain of DksA as a bone fide sensor of oxidative and nitrosative stress. We are actively testing the hypothesis that the sensing of reactive species by the DksA zinc finger promotes antioxidant and antinitrosative defenses, antibiotic resistance and the pathogenicity of Salmonella. Given that most Gram-negative bacteria express DksA orthologues with a conserved zinc finger, our investigations are likely generalizable to a variety of clinically important, and phylogenetically diverse pathogenic bacteria.
The investigations in the Vázquez-Torres lab are funded by grants from the National Institute of Health, the Veterans Administration, and the Burroughs Wellcome Fund.