Mechanisms of Salmonella Typhi Persistence

Abstract

Although human Salmonella infections are caused by a single species, Salmonella enterica, different Salmonella serovars cause distinctive clinical syndromes. Whereas non-typhoidal Salmonella (NTS) serovars typically cause self-limiting acute gastroenteritis, typhoidal serovars cause enteric fever, a severe protracted illness with systemic symptoms that can become chronic. Enteric fever accounts for a significant global burden of disease, with nearly 15 million infections and approximately 136,000 deaths annually. Currently the genetic basis for the distinct clinical outcomes caused by NTS and typhoidal Salmonella is not known. Enteric fever includes typhoid and paratyphoid fevers which are caused by S. Typhi and S. Paratyphi A, respectively; both are human host-restricted and unable to infect mice or other small animal models. Most Salmonella research has been conducted using the NTS serovar S. Typhimurium in murine models, which fail to recapitulate certain important aspects of human typhoid. The epidemiological features of acute versus persistent Salmonella infection are distinctive; inherited or acquired deficiencies in cellular immunity lead to increased susceptibility to NTS infection, but not to enteric fever. Additionally, S. Typhi and S. Paratyphi A share significant genomic differences from S. Typhimurium in the form of genomic decay and novel virulence factors. This dissertation aims to understand the underlying mechanisms that lead to the distinct clinical syndromes caused by NTS and typhoidal Salmonella infection. First, we conducted a genome-wide screen for S. Typhi virulence determinants by infecting susceptible humanized mice with a high-complexity transposon mutant library of S. Typhi. The screen identified known virulence determinants such as the Vi capsular polysaccharide and iron acquisition genes. Interestingly, certain some predicted virulence determinants were shown to be dispensible for virulence, including the typhoid toxin and Salmonella Pathogenicity Island 2 (SPI2). Given that human immune cells are required for S. Typhi infection, we next explored the interactions of both S. Typhi and S. Typhimurium with human macrophages. S. Typhi persists within human macrophages by causing minimal apoptosis unlike S. Typhimurium, which induces apoptosis in a SPI2-dependent manner. These results are consistent with our observation that typhoidal serovars S. Typhi and S. Paratyphi A have lost a significant portion of SPI2-secreted effector proteins, especially those that inhibit the NF-B pathway. Further, inhibition of NF-B was sufficient to cause macrophage apoptosis and may present a new strategy for treatment of persistent S. Typhi infection. S. Typhi also avoids inflammatory macrophage polarization and fails to induce a TH1 response in infected humanized mice. Such a response is required for NTS clearance in humans and represents another important difference between these serovars. Finally, due to the identification of iron acquisition genes as essential for S. Typhi virulence, we explored differences in iron acquisition capabilities between S. Typhi and S. Typhimurium. We found that S. Typhi is more sensitive to iron limitation, indicating adaptation to a macrophage niche that is not iron-restricted. Taken together, these chapters demonstrate the importance of employing models using typhoidal Salmonella to understand mechanisms of persistent human Salmonella infection rather than reliance on murine-NTS models.