Anaerobic Gram-Negative Bacilli

At present there are over two dozen genera of Gram-negative anaerobic bacilli. In most clinical infections, only the genera Bacteroides, Prevotella, and Fusobacterium need be considered. These genera are prevalent in the body as members of the normal flora (Fig.20-1), constituting one-third of the total anaerobic isolates from clinical specimens, and may become involved in infections throughout the body (Fig.20-2). Within the Bacteroides group, B fragilis is the most common pathogen, followed by B thetaiotaomicron and other members of the B fragilis group. Among the bile-sensitive Prevotella species, the ones most commonly encountered clinically are P melaninogenica, P oris, and P buccae. Porphyromonas species seem to be much less pathogenic except in dental infections. Fusobacterium nucleatum is the Fusobacterium species most often found as a pathogen, but F necrophorum occasionally produces serious disease. These genera contain numerous other species that rarely or never infect humans.