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

Salmonella Typhimurium serves as model for deciphering enteropathogen virulence evolution. The pathogen elicits gut inflammation by the costly expression of HilD- controlled virulence factors. Inflammation alleviates microbiota-mediated colonization resistance and thereby promotes pathogen blooms. Notably, the inflamed gut-milieu can also select for hilD mutants, which cannot elicit or maintain inflammation. However, it remains unclear why the wild-type hilD allele is dominant among natural isolates. Here, we show that microbiota transfer from healthy or recovered hosts leads to the rapid clearance of attenuated mutants and thereby contributes to the preservation of 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 microbiota leads to accumulation of hilD mutants. In contrast, in models with transient microbiota disruption, selection for attenuated hilD mutants was prevented by emergent properties of the regrowing microbiota community dominated by Lachnospirales and Oscillospirales. Strikingly, microbiota transfer from uninfected donors prevented the rise of hilD mutants after irreversible microbiota disruption. Our results establish that S. Typhimurium gut