Bacteroides are important in abscess formation (Figure 33-2). Abscesses are formed as a result of host interaction and the localization of the infecting organisms. They consist of fibrin, polymorphonuclear cells (PMN's), cellular debris, and bacteria. While the prototype infection caused by Bacteroides is an intraabdominal abscess, other infections involving Bacteroides and Prevotella are also found. These abscesses become walled-in with thick fibrous collagen capsules. Abscesses can spread locally (to surrounding tissues) or to blood stream by invasion of the vascular system and around the abscess.