Clostridiales

Clostridiales is an order of bacteria within the class Clostridia of the phylum Firmicutes (also called Bacillota). Members are typically Gram-positive, rod-shaped, and capable of forming endospores. They are predominantly anaerobic, with metabolic strategies ranging from strict anaerobes to aerotolerant organisms; motility varies among species.

The order is ecologically diverse, occurring in soil, sediments, aquatic environments, and the gastrointestinal tracts of animals, including humans. Many Clostridiales participate in fermentation, producing short-chain fatty acids, alcohols, solvents, and gases. The group includes both free-living environmental species and important commensals in animal guts; some lineages contribute to nutrient cycling and gut health, while others can be opportunistic pathogens.

Several members are notable pathogens. Clostridium species cause gas gangrene (C. perfringens), botulism (C. botulinum), tetanus (C. tetani), and food poisoning; Clostridioides difficile is a leading cause of antibiotic-associated diarrhea and colitis. In industry, certain clostridia have been explored for solvent production and biofuel research, such as acetone-butanol-ethanol fermentation by some Clostridium strains.

Taxonomy of Clostridiales has been refined by molecular methods; as a result, several taxa formerly placed in the genus Clostridium have been reclassified into Clostridioides and other genera, and the boundaries of families within the order have shifted. The group is characterized by low G+C content typical of the Firmicutes and by endospore formation, which enhances environmental resilience.