Treatment with streptomycin enhances the growth of human commensal Escherichia coli isolates in the mouse intestine, suggesting that the resident microbial community (microbiota) can inhibit the growth of invading microbes, a phenomenon known as “colonization resistance.” However, the precise mechanisms by which streptomycin treatment lowers colonization resistance remain obscure. Here we show that streptomycin treatment rendered mice more susceptible to the development of chemically induced colitis, raising the possibility that the antibiotic might lower colonization resistance by changing mucosal immune responses rather than by preventing microbe-microbe interactions. Investigation of the underlying mechanism revealed a mild inflammatory infiltrate in the cecal mucosa of streptomycin-treated mice, which was accompanied by elevated expression of Nos2, the gene that encodes inducible nitric oxide synthase. In turn, this inflammatory response enhanced the luminal growth of E. coli by nitrate respiration in a Nos2-dependent fashion. These data identify low-level intestinal inflammation as one of the factors responsible for the loss of resistance to E. coli colonization after streptomycin treatment.

SUMMARY Eradication of persistent intracellular bacterial pathogens with antibiotic therapy is often slow or incomplete. However, strategies to augment antibiotics are hampered by our poor understanding of the nutritional environment that sustains chronic infection. Here we show that the intracellular pathogen Brucella abortus survives and replicates preferentially in alternatively activated macrophages (AAM), which are more abundant during chronic infection. A metabolic shift induced by peroxisome proliferator activated receptor γ (PPARγ), which increases intracellular glucose availability, is identified as a causal mechanism promoting enhanced bacterial survival in AAM. Glucose uptake was crucial for increased replication of B. abortus in AAM, and chronic infection, as inactivation of the bacterial glucose transporter gluP reduced both intracellular survival in AAM and persistence in mice. Thus, a shift in intracellular nutrient availability induced by PPARγ promotes chronic persistence of B. abortus within AAM and targeting this pathway may aid in eradicating chronic infection.

Salmonella enterica serotype Typhi (S. Typhi) causes an estimated 21 million annual cases of typhoid fever, a severe systemic infection resulting in 200,000 to 600,000 fatalities per year (5, 37). After ingestion, S. Typhi invades the intestinal mucosa, but symptoms develop only after an average incubation period of 2 weeks (23). The relatively long incubation period of typhoid fever suggests that S. Typhi can evade or suppress detection by the innate immune system during the initial stages of infection (26,28,33). However, the virulence mechanisms that enable S. Typhi to evade components of the innate immune system early after infection have long remained elusive.One arm of the innate immune system involved in detection of invasive microbes is the complement system (9, 38). Complement deposition on the bacterial cell surface and opsonophagocytosis can be prevented by capsular polysaccharides of invasive Gramnegative pathogens, including Neisseria meningitidis, Klebsiella pneumoniae, and Escherichia coli isolates associated with extraintestinal infections (1,15,16,35). S. Typhi produces the virulence (Vi) capsular polysaccharide (8), which is encoded by the viaB locus (17). The viaB locus is a 14-kb DNA region containing genes required for the regulation (tviA), the biosynthesis (tviBCDE), and the export (vexABCDE) of the Vi capsular polysaccharide (36). In S. Typhi, the role of the Vi capsular polysaccharide in reducing complement deposition has not been convincingly demonstrated. The viaB locus of S. Typhi is located on a 134-kb DNA region, termed Salmonella pathogenicity island 7 (SPI-7) (24). SPI-7 is genetically unstable and can be lost upon laboratory passage (3,22). Clinical S. Typhi isolates expressing the Vi capsular polysaccharide tend to bind less complement on their surface in vitro than clinical isolates lacking capsule expression (20). While this report concludes that the Vi capsular polysaccharide inhibits opsonophagocytosis, the evidence is not conclusive, because it is based on comparison of nonisogenic, clinical S. Typhi isolates. Genetic differences between these clinical isolates were not defined but likely included the entire SPI-7 region. Additionally, the in vivo relevance of phenotypes attributed to the Vi capsular polysaccharide remains to be established using animal models.With the exception of higher primates, vertebrate hosts are resistant to infection with the human-adapted S. Typhi, which has prevented the use of animal models to investigate the in vivo relevance of results obtained using tissue culture. While mice orally inoculated with S. Typhi are not suited to study the development of typhoid fever, we reasoned that this animal model could be used for studying isolated steps during infection, provided that the relevant interactions were not species specific. Here we investigated the role of the Vi capsular polysaccharide on complement-mediated phagocytosis and its impact on bacterial clearance during an infection. MATERIALS AND METHODSBacterial strains and culture conditions. Vi...

Evasion of host immune responses is a prerequisite for chronic bacterial diseases; however, the underlying mechanisms are not fully understood. Here, we show that the persistent intracellular pathogen Brucella abortus prevents immune activation of macrophages by inducing CD4+CD25+ T cells to produce the anti-inflammatory cytokine interleukin-10 (IL-10) early during infection. IL-10 receptor (IL-10R) blockage in macrophages resulted in significantly higher NF-kB activation as well as decreased bacterial intracellular survival associated with an inability of B. abortus to escape the late endosome compartment in vitro. Moreover, either a lack of IL-10 production by T cells or a lack of macrophage responsiveness to this cytokine resulted in an increased ability of mice to control B. abortus infection, while inducing elevated production of pro-inflammatory cytokines, which led to severe pathology in liver and spleen of infected mice. Collectively, our results suggest that early IL-10 production by CD25+CD4+ T cells modulates macrophage function and contributes to an initial balance between pro-inflammatory and anti-inflammatory cytokines that is beneficial to the pathogen, thereby promoting enhanced bacterial survival and persistent infection.

Salmonella enterica serovar Typhi (S. Typhi) causes typhoid fever, a disseminated infection, while the closely related pathogen S. enterica serovar Typhimurium (S. Typhimurium) is associated with a localized gastroenteritis in humans. Here we investigated whether both pathogens differ in the chemotactic response they induce in neutrophils using a single-cell experimental approach. Surprisingly, neutrophils extended chemotactic pseudopodia toward Escherichia coli and S. Typhimurium, but not toward S. Typhi. Bacterial-guided chemotaxis was dependent on the presence of complement component 5a (C5a) and C5a receptor (C5aR). Deletion of S. Typhi capsule biosynthesis genes markedly enhanced the chemotactic response of neutrophils in vitro. Furthermore, deletion of capsule biosynthesis genes heightened the association of S. Typhi with neutrophils in vivo through a C5aR-dependent mechanism. Collectively, these data suggest that expression of the virulence-associated (Vi) capsular polysaccharide of S. Typhi obstructs bacterial-guided neutrophil chemotaxis.