Adult-sourced fecal microbiota transplantation ameliorates early-life antibiotic-induced intestinal immune stress in juvenile mice associated with the modulation of the TLR4/MyD88 signaling pathway

Early-life antibiotic (ABX) exposure profoundly disrupts gut microbiota assembly, triggering long-lasting intestinal immune stress. While fecal microbiota transplantation (FMT) is a promising intervention, its immunological and ecological mechanisms in the juvenile gut remain underexplored. In this study, juvenile mice were subjected to a broad-spectrum ABX cocktail for 8 days followed by 4 days of adult-sourced FMT. 16S rRNA sequencing (n = 6/group) revealed that ABX drastically altered the microbial landscape, causing a severe relative reduction of core commensals and an opportunistic expansion of Bacteroides. Subsequent molecular analyses on day 13 (n = 3/group) demonstrated that this dysbiosis significantly upregulated the mucosal TLR4/MyD88 signaling axis and effector cytokines IL-17 and IL-22. Strikingly, adult FMT administration effectively attenuated the TLR4 cascade and reduced pro-inflammatory cytokine transcript levels while immunoregulatory IL-10 transcripts remained partially elevated. Ecologically, FMT did not merely revert the gut to its naive juvenile baseline. Instead, it suppressed Bacteroides overgrowth and established a distinctly new, highly complex community enriched with putative mature, adult-signature taxa such as Parabacteroides, Tannerellaceae, and Desulfobacterota. Interestingly, the downregulation of TLR4 occurred concurrently with the engraftment of these Gram-negative, LPS-bearing taxa. Collectively, adult-sourced FMT ameliorates ABX-induced immune stress not by restoring the original microbiota, but by driving the juvenile gut towards a putatively mature, structurally distinct, and potentially immunologically tolerant ecological state.