On The Nature of Invasive Salmonella Typhi and African Salmonella Typhimurium

Abstract

Invasive typhoidal and nontyphoidal Salmonelloses are a significant global burden affecting tens of millions of individuals each year. Typhoid fever and invasive nontyphoidal Salmonellosis (iNTS) are caused by Salmonella enterica serovar Typhi (S. Typhi) and Typhimurium (S. Typhimurium) respectively but share similar disease manifestations in humans like high fever, hepato-splenomegaly and infrequent diarrhea. Despite these clinical similarities each serovar interacts differently with the human immune system. Individuals with congenital or acquired immunodeficiencies resulting in impaired Th1 immunity, such as HIV/AIDS, are more susceptible to iNTS but not typhoid fever. This suggests nontyphoidal serovars and S. Typhi differ in their propensity to initiate a Th1 immune response in infected hosts. I have found that S. Typhi-infected macrophages exhibit reduced innate immune responses to infection including macrophage apoptosis, pyroptosis and M1 activation in comparison to S. Typhimurium-infected cells. A hu-SRC-SCID humanized mouse model of infection mirrors these results. Apoptosis, pyroptosis and M1 polarization of human macrophages by S. Typhimurium is dependent upon the Salmonella pathogenicity island 2 (SPI2) type three secretion system (T3SS). While the specific effector(s) required for S. Typhimurium-induced pyroptosis and M1 activation have not been identified, intramacrophage expression of SPI2 is lower in S. Typhi compared to S. Typhimurium. Differences in intramacrophage SPI2 expression may account for differences in S. Typhi- and S. Typhimurium-induced pyroptosis and M1 activation. While S. Typhi dampens intramacrophage SPI2 expression to evade M1/Th1 responses to infection, iNTS strains exploit the compromised host immune system to cause systemic infection. Non-typhoidal S. Typhimurium is a frequent cause of bloodstream infections in children and HIV-infected adults in sub-Saharan Africa. Most isolates from African patients with bacteremia belong to a single sequence type, ST313, which is genetically distinct from gastroenteritis-associated ST19 strains such as 14028s. One important difference between ST19 strains and the sequenced ST313 strain D23580 is an increase in genomic decay in D23580, a hallmark of host-adaptation. ST313 strains were originally hypothesized as becoming more typhoid-like and thereby explaining the increased incidence of associated invasive infections. We have found that, like the enteritis-associated strain 14028s, D23580 is able to elicit an acute inflammatory response and cause enteritis in mice and rhesus macaque monkeys. However, iNTS strains are becoming more typhoid-like in their ability to survive in the environment. We have identified and demonstrated two loss-of-function mutations in D23580, not present in the ST19 strain 14028s, that impair multicellular stress resistance associated with survival outside the host. These mutations result in inactivation of the KatE stationary-phase catalase that protects high-density bacterial communities from oxidative stress and the BcsG cellulose biosynthetic enzyme required for the RDAR (red, dry and rough) colonial phenotype. Collectively, these observations suggest that African S. Typhimurium ST313 strain D23580 is becoming adapted to an anthroponotic mode of transmission while retaining the ability to infect and cause enteritis in multiple host species. We have also identified a novel, 5-gene operon in S. Typhi and S. Typhimurium that is required for virulence in mice. Previously, humanized mice susceptible to S. Typhi infection were infected with a high-density S. Typhi transposon mutant library to identify novel determinants required for invasive S. Typhi disease. From the screen, a novel, 5-gene operon was identified as required for virulence. We confirmed this requirement in additional mouse models of invasive Salmonella infection. Multiple genes in the operon have homology to carbohydrate metabolism genes. While we were unable to identify a role for these genes during growth on single carbon sources we identified a putative role for this operon in maintaining bacterial membrane stability. We measured the membrane polarization of Salmonella 5-gene operon mutant strains and found they were hyperpolarized as compared to wt further confirming a role for these genes in maintaining bacterial membrane stability. While the specific function of these genes is still unknown we continue to investigate how this novel operon promotes membrane stability in Salmonella.