Although comprising less than 0.01% of the normal human gastrointestinal microbiota, Bilophila wadsworthia is the third most common anaerobe recovered from clinical material obtained from patients with perforated and gangrenous appendicitis. Since its discovery in 1988, B. wadsworthia has been recovered from clinical specimens associated with a variety of infections, including sepsis, liver abscesses, cholecystitis, Fournier's gangrene, soft tissue abscesses, empyema, osteomyelitis, Bartholinitis, and hidradenitis suppurativa. In addition, it has been found in the saliva and vaginal fluids of asymptomatic adults and even in the periodontal pockets of dogs. The organism is a saccharolytic, fastidious, and is easily recognized by its strong catalase reaction with 15% H2O2, production of hydrogen sulfide, and growth stimulation by bile (oxgall) and pyruvate. Approximately 75% of strains are urease positive. When grown on pyruvate-containing media, > 85% of strains demonstrate beta-lactamase production. Ribosomal RNA-based phylogenetic studies show Bilophila to be a homogeneous species, most closely related to Desulfovibrio species. Both adherence to human cells and endotoxin have been observed, and preliminary work suggests that environmental iron has a role in expression of outer membrane proteins. Penicillin-binding proteins appear to mediate the organism's susceptibility to at least some beta-lactam agents, which induce spheroplast formation that results in a haze of growth on agar dilution susceptibility test plates which is difficult to interpret. Bilophilastrains are inhibited in vitro by most antibiotics.