Anaerobic bacteria will not grow when incubated with 10% CO2 in room air, but they vary in their tolerance of different levels of oxygen. Anaerobic bacteria are important commensal flora of the skin and oral, intestinal, and pelvic mucosae, and are classified according to their Gram-staining characteristics and ability to produce spores: (1) Gram-positive—cocci, non-spore-forming bacilli, and spore-forming bacilli (notably the Clostridium spp.); (2) Gram-negative—cocci and bacilli. Many anaerobic bacteria possess virulence factors that facilitate their pathogenicity (e.g. histolytic enzymes and various toxins). A putrid odour of the affected tissue or drainage is highly suggestive of an anaerobic infection, as is the presence of gas in tissues. Aside from supportive care, treatment requires drainage of abscesses and resection of devitalized tissue; and antibiotics—agents that are active against anaerobes include clindamycin, metronidazole, vancomycin, β-lactam/β-lactamase inhibitor combinations, carbapenems, moxifloxacin, tigecycline, chloramphenicol, and even macrolides.