Structural properties of short-chain carboxylic acids and alcohols relate to the molecular and physiological response of Salmonella enterica in an acidic environment.

FMT alleviates multidrug-resistant Salmonella enterica-induced diarrhea and is associated with loss of IncHI2A-associated resistance determinants in mice.

INTRODUCTION: Multidrug-resistant (MDR) Salmonella enterica (S. enterica) poses a serious threat to animal and public health because of increasingly limited treatment options. Fecal microbiota transplantation (FMT) is a potential microbiota-based intervention; however, its effects on MDR Salmonella infection and pathogen-associated antibiotic resistance gene (ARG) dynamics remain unclear. METHODS: A murine diarrhea model was established using the clinical MDR S. enterica isolate P174, and infected mice were treated with FMT. Clinical symptoms, intestinal pathology, transcriptional inflammatory responses, gut microbiota composition, and ARG profiles of recovered Salmonella isolates were evaluated. Whole-genome sequencing was used to characterize resistance determinants, and the stability of ARGs and IncHI2A backbone markers was further assessed during 19 in vitro passages. RESULTS: FMT reduced diarrhea, promoted body weight recovery, and alleviated intestinal tissue injury and inflammatory cell infiltration. Colonic expression of Tnf, Il1b, and Il6 decreased, whereas Il10 expression increased. FMT was also associated with partial recovery of gut microbial diversity, increased relative abundances of Lactobacillus, Bifidobacterium, and other commensal anaerobic taxa, and reduced Salmonella abundance. Whole-genome sequencing showed that bla OXA-1, floR, oqxA, and oqxB were co-localized on an IncHI2A-associated plasmid sequence. Loss of these resistance determinants increased over time in isolates recovered from FMT-treated mice, whereas no loss of the four ARGs or the IncHI2A backbone markers repB and parB was detected during 19 in vitro passages. Among isolates showing simultaneous loss of all four ARGs, nearly all also lacked detectable repB and parB, whereas isolates with partial ARG loss retained both markers. These patterns were consistent with both backbone-associated loss and resistance-region deletion or rearrangement. Most ARG-loss isolates showed reduced antimicrobial resistance. DISCUSSION: FMT alleviated MDR S. enterica-induced intestinal disease and was associated with partial recovery of gut microbiota characteristics and increased instability and loss of IncHI2A-associated resistance determinants in vivo. These findings suggest a potential association between intestinal microbial ecological changes and altered maintenance patterns of resistance-associated genetic elements in MDR S. enterica.