Multiple histidines in the periplasmic domain of the Salmonella enterica sensor kinase SsrA enhance signaling in response to extracellular acidification.

The two-component regulatory system SsrA-SsrB in Salmonella enterica controls expression of a virulence gene program required for intracellular survival in host cells. SsrA signaling is induced within the acidic host vacuole in which the bacteria reside; however, the mechanism by which SsrA senses this intracellular environment is unknown. Here, we show that the periplasmic sensor domain of SsrA is enriched in histidine residues that increase SsrA signaling below external pH of 6. While no single histidine accounted for the full acid-responsiveness of SsrA, we localized the acid-responsiveness principally to five histidines in the C-terminal end of the periplasmic sensor domain, with input from additional histidines in the N-terminal end of the senor. A sensor mutant lacking critical pH-responsive histidines was defective for acid-promoted activity, yet retained basal activity similar to wild type at neutral pH, indicating that the role of these histidines is to enhance signaling in response to acidification. In support of this, a pH-blind mutant was insensitive to the vacuole acidification blocking activity of bafilomycin, and was attenuated for competitive fitness during infection of mice. Our data demonstrate that SsrA contains a histidine-rich periplasmic sensor that enhances signaling in response to the innate host defense of vacuolar acidification.

Host defense peptide resistance contributes to colonization and maximal intestinal pathology by Crohn's disease-associated adherent-invasive Escherichia coli.

Host defense peptides secreted by colonocytes and Paneth cells play a key role in innate host defenses in the gut. In Crohn's disease, the burden of tissue-associated Escherichia coli commonly increases at epithelial surfaces where host defense peptides concentrate, suggesting that this bacterial population might actively resist this mechanism of bacterial killing. Adherent-invasive E. coli (AIEC) is associated with Crohn's disease; however, the colonization determinants of AIEC in the inflamed gut are undefined. Here, we establish that host defense peptide resistance contributes to host colonization by Crohn's-associated AIEC. We identified a plasmid-encoded genomic island (called PI-6) in AIEC strain NRG857c that confers high-level resistance to α-helical cationic peptides and α- and ß-defensins. Deletion of PI-6 sensitized strain NRG857c to these host defense molecules, reduced its competitive fitness in a mouse model of infection, and attenuated its ability to induce cecal pathology. This phenotype is due to two genes in PI-6, arlA, which encodes a Mig-14 family protein implicated in defensin resistance, and arlC, an OmpT family outer membrane protease. Implicit in these findings are new bacterial targets whose inhibition might limit AIEC burden and disease in the gut.

Persistent infection with Crohn's disease-associated adherent-invasive Escherichia coli leads to chronic inflammation and intestinal fibrosis.

Crohn's disease is a chronic inflammatory condition of the gastrointestinal tract in which alterations to the bacterial community contribute to disease. Adherent-invasive Escherichia coli are associated with human Crohn's disease; however, their role in intestinal immunopathology is unclear because of the lack of an animal model compatible with chronic timescales. Here we establish chronic adherent-invasive Escherichia coli infection in streptomycin-treated conventional mice (CD1, DBA/2, C3H, 129e and C57BL/6), enabling the study of host response and immunopathology. Adherent-invasive Escherichia coli induces an active T-helper 17 response, heightened levels of proinflammatory cytokines and fibrotic growth factors, with transmural inflammation and fibrosis. Depletion of CD8+ T cells increases caecal bacterial load, pathology and intestinal fibrosis in C57BL/6 mice, suggesting a protective role. Our findings provide evidence that chronic adherent-invasive Escherichia coli infections result in immunopathology similar to that seen in Crohn's disease. With this model, research into the host and bacterial genetics associated with adherent-invasive Escherichia coli-induced disease becomes more widely accessible.