Salmonella T3SS-2 virulence enhances gut-luminal colonization by enabling chemotaxis-dependent exploitation of intestinal inflammation.

Salmonella Typhimurium (S.Tm) utilizes the chemotaxis receptor Tsr to exploit gut inflammation. However, the characteristics of this exploitation and the mechanism(s) employed by the pathogen to circumvent antimicrobial effects of inflammation are poorly defined. Here, using different naturally occurring S.Tm strains (SL1344 and 14028) and competitive infection experiments, we demonstrate that type-three secretion system (T3SS)-2 virulence is indispensable for the beneficial effects of Tsr-directed chemotaxis. The removal of the 14028-specific prophage Gifsy3, encoding virulence effectors, results in the loss of the Tsr-mediated fitness advantage in that strain. Surprisingly, without T3SS-2 effector secretion, chemotaxis toward the gut epithelium using Tsr becomes disadvantageous for either strain. Our findings reveal that luminal neutrophils recruited as a result of NLRC4 inflammasome activation locally counteract S.Tm cells exploiting the byproducts of the host immune response. This work highlights a mechanism by which S.Tm exploitation of gut inflammation for colonization relies on the coordinated effects of chemotaxis and T3SS activities.

The microbiota conditions a gut milieu that selects for wild-type Salmonella Typhimurium virulence.

Salmonella Typhimurium elicits gut inflammation by the costly expression of HilD-controlled virulence factors. This inflammation alleviates colonization resistance (CR) mediated by the microbiota and thereby promotes pathogen blooms. However, the inflamed gut-milieu can also select for hilD mutants, which cannot elicit or maintain inflammation, therefore causing a loss of the pathogen's virulence. This raises the question of which conditions support the maintenance of virulence in S. Typhimurium. Indeed, it remains unclear why the wild-type hilD allele is dominant among natural isolates. Here, we show that microbiota transfer from uninfected or recovered hosts leads to rapid clearance of hilD mutants that feature attenuated virulence, and thereby contributes to the preservation of the virulent S. Typhimurium genotype. Using mouse models featuring a range of microbiota compositions and antibiotic- or inflammation-inflicted microbiota disruptions, we found that irreversible disruption of the microbiota leads to the accumulation of hilD mutants. In contrast, in models with a transient microbiota disruption, selection for hilD mutants was prevented by the regrowing microbiota community dominated by Lachnospirales and Oscillospirales. Strikingly, even after an irreversible microbiota disruption, microbiota transfer from uninfected donors prevented the rise of hilD mutants. Our results establish that robust S. Typhimurium gut colonization hinges on optimizing its manipulation of the host: A transient and tempered microbiota perturbation is favorable for the pathogen to both flourish in the inflamed gut and also minimize loss of virulence. Moreover, besides conferring CR, the microbiota may have the additional consequence of maintaining costly enteropathogen virulence mechanisms.

Intraluminal neutrophils limit epithelium damage by reducing pathogen assault on intestinal epithelial cells during Salmonella gut infection.

Recruitment of neutrophils into and across the gut mucosa is a cardinal feature of intestinal inflammation in response to enteric infections. Previous work using the model pathogen Salmonella enterica serovar Typhimurium (S.Tm) established that invasion of intestinal epithelial cells by S.Tm leads to recruitment of neutrophils into the gut lumen, where they can reduce pathogen loads transiently. Notably, a fraction of the pathogen population can survive this defense, re-grow to high density, and continue triggering enteropathy. However, the functions of intraluminal neutrophils in the defense against enteric pathogens and their effects on preventing or aggravating epithelial damage are still not fully understood. Here, we address this question via neutrophil depletion in different mouse models of Salmonella colitis, which differ in their degree of enteropathy. In an antibiotic pretreated mouse model, neutrophil depletion by an anti-Ly6G antibody exacerbated epithelial damage. This could be linked to compromised neutrophil-mediated elimination and reduced physical blocking of the gut-luminal S.Tm population, such that the pathogen density remained high near the epithelial surface throughout the infection. Control infections with a ssaV mutant and gentamicin-mediated elimination of gut-luminal pathogens further supported that neutrophils are protecting the luminal surface of the gut epithelium. Neutrophil depletion in germ-free and gnotobiotic mice hinted that the microbiota can modulate the infection kinetics and ameliorate epithelium-disruptive enteropathy even in the absence of neutrophil-protection. Together, our data indicate that the well-known protective effect of the microbiota is augmented by intraluminal neutrophils. After antibiotic-mediated microbiota disruption, neutrophils are central for maintaining epithelial barrier integrity during acute Salmonella-induced gut inflammation, by limiting the sustained pathogen assault on the epithelium in a critical window of the infection.

Differences in carbon metabolic capacity fuel co-existence and plasmid transfer between Salmonella strains in the mouse gut.

Antibiotic resistance plasmids can be disseminated between different Enterobacteriaceae in the gut. Here, we investigate how closely related Enterobacteriaceae populations with similar nutrient needs can co-bloom in the same gut and thereby facilitate plasmid transfer. Using different strains of Salmonella Typhimurium (S.Tm SL1344 and ATCC14028) and mouse models of Salmonellosis, we show that the bloom of one strain (i.e., recipient) from very low numbers in a gut pre-occupied by the other strain (i.e., donor) depends on strain-specific utilization of a distinct carbon source, galactitol or arabinose. Galactitol-dependent growth of the recipient S.Tm strain promotes plasmid transfer between non-isogenic strains and between E. coli and S.Tm. In mice stably colonized by a defined microbiota (OligoMM12), galactitol supplementation similarly facilitates co-existence of two S.Tm strains and promotes plasmid transfer. Our work reveals a metabolic strategy used by Enterobacteriaceae to expand in a pre-occupied gut and provides promising therapeutic targets for resistance plasmids spread.