Bacteroides

Bacteroides is a genus of Gram-negative, obligate anaerobic, non-spore-forming bacteria. They are among the most abundant constituents of the human gut microbiota, and species occur in the oral cavity and the lower reproductive tract in some individuals.

Most Bacteroides species are commensals that help break down complex polysaccharides and fibers, producing short-chain fatty acids such as acetate and propionate that contribute to colon health and energy metabolism. Some species, particularly within the Bacteroides fragilis group, interact with the host immune system, including capsular polysaccharides that modulate regulatory T-cell responses.

Bacteroides fragilis and related species can become opportunistic pathogens when mucosal barriers are breached, causing intra-abdominal infections, abscesses, pelvic infections, and bacteremia. Bacteroides species are among the more resistant anaerobes in clinical settings; many strains produce beta-lactamases, limiting the effectiveness of some penicillins and cephalosporins. Treatment often involves anaerobic coverage with metronidazole, carbapenems, or beta-lactam/beta-lactamase inhibitor combinations, guided by susceptibility testing.

Taxonomically, Bacteroides belongs to the phylum Bacteroidota and the family Bacteroidaceae. Culture requires strict anaerobic conditions, and identification in clinical labs combines Gram staining, colony morphology, and biochemical tests, with molecular methods increasingly used. Notable species include Bacteroides fragilis, a prominent opportunistic pathogen, as well as Bacteroides thetaiotaomicron and Bacteroides vulgatus, which are common gut commensals and model organisms in microbiome research.