Most mucosal surfaces of the mammalian body are colonized by microbial communities (“microbiota”). A high density of commensal microbiota inhabits the intestine and shields from infection (“colonization resistance”). The virulence strategies allowing enteropathogenic bacteria to successfully compete with the microbiota and overcome colonization resistance are poorly understood. Here, we investigated manipulation of the intestinal microbiota by the enteropathogenic bacterium Salmonella enterica subspecies 1 serovar Typhimurium (S. Tm) in a mouse colitis model: we found that inflammatory host responses induced by S. Tm changed microbiota composition and suppressed its growth. In contrast to wild-type S. Tm, an avirulent invGsseD mutant failing to trigger colitis was outcompeted by the microbiota. This competitive defect was reverted if inflammation was provided concomitantly by mixed infection with wild-type S. Tm or in mice (IL10−/−, VILLIN-HACL4-CD8) with inflammatory bowel disease. Thus, inflammation is necessary and sufficient for overcoming colonization resistance. This reveals a new concept in infectious disease: in contrast to current thinking, inflammation is not always detrimental for the pathogen. Triggering the host's immune defence can shift the balance between the protective microbiota and the pathogen in favour of the pathogen.

Intestinal dendritic cells (DCs) are believed to sample and present commensal bacteria to the gut-associated immune system to maintain immune homeostasis. How antigen sampling pathways handle intestinal pathogens remains elusive. We present a murine colitogenic Salmonella infection model that is highly dependent on DCs. Conditional DC depletion experiments revealed that intestinal virulence of S. Typhimurium SL1344 ΔinvG mutant lacking a functional type 3 secretion system-1 (ΔinvG)critically required DCs for invasion across the epithelium. The DC-dependency was limited to the early phase of infection when bacteria colocalized with CD11c+CX3CR1+ mucosal DCs. At later stages, the bacteria became associated with other (CD11c−CX3CR1−) lamina propria cells, DC depletion no longer attenuated the pathology, and a MyD88-dependent mucosal inflammation was initiated. Using bone marrow chimeric mice, we showed that the MyD88 signaling within hematopoietic cells, which are distinct from DCs, was required and sufficient for induction of the colitis. Moreover, MyD88-deficient DCs supported transepithelial uptake of the bacteria and the induction of MyD88-dependent colitis. These results establish that pathogen sampling by DCs is a discrete, and MyD88-independent, step during the initiation of a mucosal innate immune response to bacterial infection in vivo.

Escherichia coli strain O157 produces an O-antigen with the repeating tetrasaccharide unit ␣-D-PerNAc-␣-L-Fuc-␤-D-Glc-␣-D-GalNAc, preassembled on undecaprenyl pyrophosphate (Und-P-P). These studies were conducted to determine whether the biosynthesis of the lipid-linked repeating tetrasaccharide was initiated by the formation of GalNAc-P-P-Und by WecA. 3 H]GlcNAc-P-P-Und was reformed when membranes from strain O157 were incubated with exogenous [ 3 H]GalNAc-P-P-Und. The inability of Z3206 to complement the loss of the gne gene in the expression of the Campylobacter jejuni N-glycosylation system in E. coli indicated that it does not function as a UDP-GlcNAc/UDPGalNAc epimerase. Based on these results, GalNAc-P-P-Und is synthesized reversibly by a novel GlcNAc-P-P-Und epimerase after the formation of GlcNAc-P-P-Und by WecA in E. coli O157.The Escherichia coli cell envelope contains an inner plasma membrane, a stress-bearing peptidoglycan layer, and an asymmetric outer membrane consisting of a phospholipid inner monolayer and an outer monolayer composed of bacterial lipopolysaccharide (LPS).2 LPS contains three components, the lipid A anchor, the 3-deoxy-D-manno-oct-2-ulosonic acid-containing core, and the O-antigen region (see Refs.