β-hydroxybutyrate modulates enteric pathogen susceptibility through regulation of commensal bacteria and intestinal Th17 responses

T helper 17 (Th17) cells are a critical T lymphocyte subset involved in mucosal immunity and host defense against enteric pathogens. Although ketogenic diets (KD) and the major ketone body {beta}-hydroxybutyrate (BHB) reshape gut microbiota and suppress Th17 responses under defined diet conditions, it remains unclear whether elevation of BHB alone, independent of dietary macronutrient composition and systematic metabolic shift, is sufficient to remodel Th17-inducing commensals and alter host susceptibility to enteric infection. Here, we used 1,3-butanediol (BD), a precursor metabolized to BHB independently of KD, to elevate systemic BHB levels in mice. BD treatment significantly reduced the frequency of ileal Th17 cells, as assessed by flow cytometry for Th17 markers IL-17A and ROR{gamma}t. 16S rRNA gene sequencing revealed that BD altered gut microbial community structure, as indicated by beta-diversity analysis based on Bray-Curtis dissimilarity, and reduced Shannon diversity and evenness. Linear discriminant analysis effect size identified segmented filamentous bacteria (SFB) as significantly decreased in the ileum following BD treatment, and SFB abundance positively correlated with Th17 markers. Microbiota transplantation demonstrated that BD-shaped microbiota was sufficient to suppress Th17 responses in recipient mice, accompanied by reduced SFB abundance. In a Citrobacter rodentium infection model, BD treatment was associated with increased pathogen burden, and fecal C. rodentium levels were negatively correlated with SFB abundance. Together, these results indicate that BD-induced elevation of BHB reshapes commensal microbiota, including decreasing SFB levels, resulting in dampened Th17 responses and increased susceptibility to enteric infection. IMPORTANCEDiet is a key determinant of gut microbial composition and mucosal immune function, yet the microbial mechanisms linking how diet-mediated changes to metabolism regulate immune responses remain incompletely understood. Th17 cells play central roles in both protective mucosal immunity and inflammatory pathology, making them a critical target of immunometabolic regulation. In this study, we show that {beta}-hydroxybutyrate (BHB), generated independently of diet, suppresses intestinal Th17 responses by reshaping the gut microbiota, reducing SFB levels, a potent Th17-inducing murine commensal. We further demonstrate that BHB-associated microbiota changes are linked to increased susceptibility to enteric infection. This work provides a mechanistic framework illustrating how metabolic state can influence host immunity through selective effects on commensal microbes. These findings inform future studies of microbiota-mediated immune regulation.