Genome-wide Analysis of Salmonella enterica serovar Typhi in Humanized Mice Reveals Key Virulence Features.

Salmonella enterica serovar Typhi causes typhoid fever only in humans. Murine infection with S. Typhimurium is used as a typhoid model, but its relevance to human typhoid is limited. Non-obese diabetic-scid IL2rγnull mice engrafted with human hematopoietic stem cells (hu-SRC-SCID) are susceptible to lethal S. Typhi infection. In this study, we use a high-density S. Typhi transposon library in hu-SRC-SCID mice to identify virulence loci using transposon-directed insertion site sequencing (TraDIS). Vi capsule, lipopolysaccharide (LPS), and aromatic amino acid biosynthesis were essential for virulence, along with the siderophore salmochelin. However, in contrast to the murine S. Typhimurium model, neither the PhoPQ two-component system nor the SPI-2 pathogenicity island was required for lethal S. Typhi infection, nor was the CdtB typhoid toxin. These observations highlight major differences in the pathogenesis of typhoid and non-typhoidal Salmonella infections and demonstrate the utility of humanized mice for understanding the pathogenesis of a human-specific pathogen.

Iron and citrate export by a major facilitator superfamily pump regulates metabolism and stress resistance in Salmonella Typhimurium.

The efficacy of antibiotics and host defenses has been linked to the metabolic and redox states of bacteria. In this study we report that a stress-induced export pump belonging to the major facilitator superfamily effluxes citrate and iron from the enteric pathogen Salmonella Typhimurium to arrest growth and ameliorate the effects of antibiotics, hydrogen peroxide, and nitric oxide. The transporter, formerly known as MdtD, is now designated IceT (iron citrate efflux transporter). Iron efflux via an iron-chelating tricarboxylic acid cycle intermediate provides a direct link between aerobic metabolism and bacterial stress responses, representing a unique mechanism of resistance to host defenses and antimicrobial agents of diverse classes.

Functional analysis of the RdxA and RdxB nitroreductases of Campylobacter jejuni reveals that mutations in rdxA confer metronidazole resistance.

Campylobacter jejuni is a leading cause of gastroenteritis in humans and a commensal bacterium of the intestinal tracts of many wild and agriculturally significant animals. We identified and characterized a locus, which we annotated as rdxAB, encoding two nitroreductases. RdxA was found to be responsible for sensitivity to metronidazole (Mtz), a common therapeutic agent for another epsilonproteobacterium, Helicobacter pylori. Multiple, independently derived mutations in rdxA but not rdxB resulted in resistance to Mtz (Mtz(r)), suggesting that, unlike the case in H. pylori, Mtz(r) might not be a polygenic trait. Similarly, Mtz(r) C. jejuni was isolated after both in vitro and in vivo growth in the absence of selection that contained frameshift, point, insertion, or deletion mutations within rdxA, possibly revealing genetic variability of this trait in C. jejuni due to spontaneous DNA replication errors occurring during normal growth of the bacterium. Similar to previous findings with H. pylori RdxA, biochemical analysis of C. jejuni RdxA showed strong oxidase activity, with reduction of Mtz occurring only under anaerobic conditions. RdxB showed similar characteristics but at levels lower than those for RdxA. Genetic analysis confirmed that rdxA and rdxB are cotranscribed and induced during in vivo growth in the chick intestinal tract, but an absence of these genes did not strongly impair C. jejuni for commensal colonization. Further studies indicate that rdxA is a convenient locus for complementation of mutants in cis. Our work contributes to the growing knowledge of determinants contributing to susceptibility to Mtz (Mtz(s)) and supports previous observations of the fundamental differences in the activities of nitroreductases from epsilonproteobacteria.

Characterization of two putative cytochrome c peroxidases of Campylobacter jejuni involved in promoting commensal colonization of poultry.

Campylobacter jejuni is a leading cause of bacterial gastroenteritis in humans throughout the world, but infection of animals, especially poultry, results in a commensal colonization of the intestines. We previously found that a mutant lacking docA, which encodes a putative cytochrome c peroxidase (CCP), demonstrates up to a 10(5)-fold reduction in colonization of the chick cecum compared to wild-type C. jejuni strain 81-176. Predictions from genomic sequences identified CJJ0382 as a second locus in C. jejuni encoding a CCP, making the bacterium unusual in having two putative CCPs. To understand what advantages are imparted by having two putative CCPs, we compared the colonization requirements of C. jejuni mutants lacking DocA or Cjj0382. Unlike the DeltadocA mutant, a DeltaCJJ0382 mutant demonstrates a maximal 50-fold colonization defect that is dependent on the inoculum dose. The colonization differences of mutants lacking DocA or Cjj0382 suggest that the two predicted CCPs are unlikely to perform redundant functions during in vivo growth. In the characterizations of DocA and Cjj0382, we found that they are stable periplasmic proteins with an apparent heme-dependent peroxidase activity, which are characteristics of bacterial CCPs. However, the peroxidase activities of the proteins do not appear to contribute to resistance to hydrogen peroxide. Instead, we found that resistance to hydrogen peroxide in C. jejuni is mostly attributed to the cytoplasmic catalase KatA. Our data suggest that DocA and Cjj0382 have characteristics of CCPs but likely perform different physiological functions for the bacterium in colonization that are not related to resisting oxidative stress.