Bacteroidales

Bacteroidales is an order of Gram-negative, non-spore-forming bacteria within the phylum Bacteroidota. Members are predominantly anaerobic, pleomorphic rods that inhabit the gastrointestinal tracts of humans and other animals, as well as environmental niches such as soil and seawater. The order contains several families, including Bacteroidaceae, Prevotellaceae, Tannerellaceae, and Rikenellaceae, with multiple genera that vary in ecology and pathogenic potential.

Physiologically, most Bacteroidales are strict or near-strict anaerobes and non-motile. They metabolize a wide range of carbohydrates, especially complex polysaccharides, using diverse carbohydrate-active enzymes. In the gut, they contribute to the breakdown of dietary fibers and host glycans, producing short-chain fatty acids that support colon health and energy balance.

Ecology and clinical relevance: In healthy individuals, Bacteroidales are dominant members of the gut microbiota and participate in nutrient processing and colonization resistance against pathogens. When translocated outside the gut or during dysbiosis, some members—especially the Bacteroides fragilis group—can cause serious infections such as abscesses and bacteremia. Many species carry beta-lactamases or other resistance genes, complicating treatment. Some genera, such as Porphyromonas and Tannerella, are associated with periodontal disease.

Notable features and research: The genomes of Bacteroidales encode extensive CAZyme repertoires, reflecting adaptation to diverse glycans. Taxonomy continues to be refined as molecular methods reveal deeper diversity.