Salmonella Typhimurium metabolism in the murine host and importance to virulence

© 2014 Dr. Timothy Alexander ScottThe bacterial pathogen Salmonella enterica is responsible for considerable global morbidity and mortality, being the cause of several enteric and systemic diseases, including typhoid fever. Non- typhoidal Salmonella (NTS) serovars such as Typhimurium cause gastroenteritis in immunocompetent individuals, but in the context of HIV-co-infection can cause invasive disease with high fatality rates. Successive generations of antimicrobials have become ineffective against S. enterica pathogens due to the widespread development of resistance, and new drugs are critically needed. In addition, vaccines against typhoid fever have sub-optimal efficacy and no human NTS vaccines are currently available. Although the in vitro metabolism of S. enterica is relatively well defined, little is known about the specific nutrients that the pathogen consumes in its hosts, and the metabolic mechanisms by which S. enterica utilise these nutrients. Therefore, the central aim of this study was to investigate and characterise S. enterica serovar Typhimurium metabolism in the murine host. Attention was focused on two areas of S. Typhimurium metabolism which have been speculated to contribute to bacterial virulence: methylglyoxal detoxification and central carbon (sugar) catabolism. It was anticipated that this study may contribute to better defining the metabolic requirements of S. Typhimurium during infection and disease, and present opportunities for the identification of potential drug targets and metabolically-attenuated strains amenable to use as live vaccines. Several studies have ventured that methylglyoxal detoxification mechanisms are required for survival of bacterial pathogens in the mammalian host, but this hypothesis has not been thoroughly tested. In the current study, although S. Typhimurium mutants defective in the glutathione-dependent glyoxalase system (GDGS) or Kef-mediated potassium efflux were highly sensitive to methylglyoxal, they were not attenuated for intracellular replication and growth in mice, suggesting that these methylglyoxal detoxification mechanisms are not required for S.Typhimurium pathogenesis in the mammalian host and are not suitable targets for antimicrobial therapy against S. Typhimurium disease. While others have reported that the Embden-Meyerhof pathway (EMP) and the ability to utilise glucose are required for S. Typhimurium virulence in mice, the importance of other sugars and carbon catabolic pathways to S. enterica virulence is unclear. In this study, S. Typhimurium mutants blocked in several sugar catabolic pathways including the Entner-Doudoroff pathway (EDP) were found not to be attenuated for intracellular growth or fulminant infection of mice, demonstrating that gluconate, glucuronate, galacturonate are not essential carbon sources for S. Typhimurium in vivo. However, evidence was presented which suggested that the ability to utilise gluconate and glucose is required for optimal shedding of the pathogen in the murine faeces, revealing potential strategies for reducing the faecal-oral transmission of S. Typhimurium. EMP/EDP double mutants showed greater attenuation in mice than a EMP mutant, suggesting that the EMP mutant utilises the EDP in order to facilitate it’s modest growth in vivo. These findings demonstrated the functional redundancy of S. Typhimurium metabolism and suggested that combinational drug therapies targeting several bacterial pathways concurrently might be a viable option for treating S. Typhimurium disease. The S. Typhimurium EMP/EDP mutant TAS2010 was found to provide increased, long-term protection from virulent infection in a murine typhoid vaccination model than the prototypical aro-negative vaccine strain BRD509. Given that the lack of effective vaccines against S. enterica pathogens can be largely attributed to an insufficient understanding of the host immune response to the pathogen, the immunological response to the TAS2010 vaccine strain was characterised in mice to understand the mechanisms responsible for the increased protective capabilities of this strain. In comparison to BRD509, TAS2010 was found to replicate to higher numbers in murine organs and induce an increased immune response in the form of increased interferon-gamma secretion by splenic CD4+ T cells. Evidence was presented which suggested that the protection provided by TAS2010 is less reliant on T cells and more dependent on a CD4-CD8-Thy1+ lymphocyte subset, probably Thy1+ NK cells. In conclusion, this study has enhanced the understanding of S. Typhimurium metabolism in the murine host and introduced a live vaccine strain with improved protective and immunogenic properties