Gram negative anaerobes are some of the most numerous organisms inhabiting the GI tract, oral cavity, and female genital tract. These organisms make up a portion of the normal flora in these areas, and are responsible in part as a protection against invading organisms. Therefore, it is unusual for them to cause disease, unless they are out of their normal habitat.

Important Gram Negative Anaerobes
Table 33.1 provides a list of clinicaly important gram negative anaerobes. The most important of the gram negative anaerobes are the Bacteroides group that inhabit the colon. These obligately anaerobic organisms can number as high as 1011 per gram of feces. Ten species of Bacteroides are defined, with B. fragilis, B. thetaiotaomicron, and B. distasonis being the most numerous.

A second group of gram negative anaerobes that were previously called Bacteroides are the Prevotella group. There are Prevotella species found in both the oral and pelvic regions. The most important of the pelvic species are P. bivia and P. disiens (formerly B. bivius and B. disiens). The most important species in the oral cavity are P. melanogenica (formerly B. melanogenicus).

The organisms listed above are all obligately anaerobic, but unlike some fastidious anaerobes, they are able to tolerate long exposures to oxygen without being killed. They are the principle organisms discussed in this lecture.

Two other important genuses of anaerobes, Veillonella and Fusobacteria, that are found principally in the oral cavity and G.I. tract, will be discussed at the end of the lecture.