<title>Abstract</title>
Centenarians, or individuals who have lived more than a century, represent the ultimate model of successful longevity associated with decreased susceptibility to ageing-associated illness and chronic inflammation. The gut microbiota is considered to be a critical determinant of human health and longevity. Here we show that centenarians (average 107 yo) have a distinct gut microbiome enriched in microbes capable of generating unique secondary bile acids, including iso-, 3-oxo-, and isoallo-lithocholic acid (LCA), as compared to elderly (85-89 yo) and young (21-55 yo) controls. Among these bile acids, the biosynthetic pathway for isoalloLCA had not been described previously. By screening 68 bacterial isolates from a centenarian’s faecal microbiota, we identified <italic>Parabacteroides merdae</italic> and <italic>Odoribacteraceae</italic> strains as effective producers of isoalloLCA. Furthermore, we generated and tested mutant strains of <italic>P. merdae</italic> to show that the enzymes 5α-reductase (5AR) and 3β-hydroxysteroid dehydrogenase (3βHSDH) were responsible for isoalloLCA production. This secondary bile acid derivative exerted the most potent antimicrobial effects among the tested bile acid compounds against gram-positive (but not gram-negative) multidrug-resistant pathogens, including <italic>Clostridioides difficile</italic> and vancomycin-resistant <italic>Enterococcus faecium</italic>. These findings suggest that specific bile acid metabolism may be involved in reducing the risk of pathobiont infection, thereby potentially contributing to longevity.