Nucleotide-binding oligomerization domain protein (NOD)1 and NOD2 participate in signaling pathways that detect pathogen-induced processes, such as the presence of peptidoglycan fragments in the host cell cytosol, as danger signals. Recent work suggests that peptidoglycan fragments activate NOD1 indirectly, through activation of the small Rho GTPase Ras-related C3 botulinum toxin substrate 1 (RAC1). Excessive activation of small Rho GTPases by virulence factors of enteric pathogens also triggers the NOD1 signaling pathway. Many enteric pathogens use virulence factors that alter the activation state of small Rho GTPases, thereby manipulating the host cell cytoskeleton of intestinal epithelial cells to promote bacterial attachment or entry. These data suggest that the NOD1 signaling pathway in intestinal epithelial cells provides an important sentinel function for detecting 'breaking and entering' by enteric pathogens.

Intestinal inflammation changes the luminal habitat for microbes through mechanisms that have not been fully resolved. We noticed that the FepE regulator of very long O-antigen chain assembly in the enteric pathogen Salmonella enterica serotype Typhimurium (S. Typhimurium) conferred a luminal fitness advantage in the mouse colitis model. However, a fepE mutant was not defective for survival in tissue, resistance to complement or resistance to polymyxin B. We performed metabolite profiling to identify changes in the luminal habitat that accompany S. Typhimurium-induced colitis. This analysis suggested that S. Typhimurium-induced colitis increased the luminal concentrations of total bile acids. A mutation in fepE significantly reduced the minimal inhibitory concentration (MIC) of S. Typhimurium for bile acids in vitro. Oral administration of the bile acid sequestrant cholestyramine resin lowered the concentrations of total bile acids in colon contents during S. Typhimurium infection and significantly reduced the luminal fitness advantage conferred by the fepE gene in the mouse colitis model. Collectively, these data suggested that very long O-antigen chains function in bile acid resistance of S. Typhimurium, a property conferring a fitness advantage during luminal growth in the inflamed intestine.

The invasion-associated type III secretion system (T3SS-1) of Salmonella enterica serotype Typhimurium (S. Typhimurium) activates the transcription factor NF-κB in tissue culture cells and induces inflammatory responses in animal models through unknown mechanisms. Here we show that bacterial delivery or ectopic expression of SipA, a T3SS-1-translocated protein, led to the activation of the NOD1/NOD2 signaling pathway and consequent RIP2-mediated induction of NF-κB-dependent inflammatory responses. SipA-mediated activation of NOD1/NOD2 signaling was independent of bacterial invasion in vitro but required an intact T3SS-1. In the mouse colitis model, SipA triggered mucosal inflammation in wild-type mice but not in NOD1/NOD2-deficient mice. These findings implicate SipA-driven activation of the NOD1/NOD2 signaling pathway as a mechanism by which the T3SS-1 induces inflammatory responses in vitro and in vivo.

Delivery of microbial products into the mammalian cell cytosol by bacterial secretion systems is a strong stimulus for triggering pro-inflammatory host responses. Here we show that Salmonella enterica serovar Typhi (S. Typhi), the causative agent of typhoid fever, tightly regulates expression of the invasion-associated type III secretion system (T3SS-1) and thus fails to activate these innate immune signaling pathways. The S. Typhi regulatory protein TviA rapidly repressed T3SS-1 expression, thereby preventing RAC1-dependent, RIP2-dependent activation of NF-κB in epithelial cells. Heterologous expression of TviA in S. enterica serovar Typhimurium (S. Typhimurium) suppressed T3SS-1-dependent inflammatory responses generated early after infection in animal models of gastroenteritis. These results suggest that S. Typhi reduces intestinal inflammation by limiting the induction of pathogen-induced processes through regulation of virulence gene expression.

Our innate immune system distinguishes microbes from self by detecting conserved pathogen-associated molecular patterns. However, these are produced by all microbes, regardless of their pathogenic potential. To distinguish virulent microbes from those with lower disease-causing potential the innate immune system detects conserved pathogen-induced processes, such as the presence of microbial products in the host cytosol, by mechanisms that are not fully resolved. Here we show that NOD1 senses cytosolic microbial products by monitoring the activation state of small Rho GTPases. Activation of RAC1 and CDC42 by bacterial delivery or ectopic expression of SopE, a virulence factor of the enteric pathogen Salmonella, triggered the NOD1 signalling pathway, with consequent RIP2 (also known as RIPK2)-mediated induction of NF-κB-dependent inflammatory responses. Similarly, activation of the NOD1 signalling pathway by peptidoglycan required RAC1 activity. Furthermore, constitutively active forms of RAC1, CDC42 and RHOA activated the NOD1 signalling pathway. Our data identify the activation of small Rho GTPases as a pathogen-induced process sensed through the NOD1 signalling pathway.

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 (AAMs), 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 AAMs. Glucose uptake was crucial for increased replication of B. abortus in AAMs, and for chronic infection, as inactivation of the bacterial glucose transporter gluP reduced both intracellular survival in AAMs and persistence in mice. Thus, a shift in intracellular nutrient availability induced by PPARγ promotes chronic persistence of B. abortus within AAMs, and targeting this pathway may aid in eradicating chronic infection.