The Bacteroides fragilis cell envelope: quarterback, linebacker, coach-or all three?

Bacteroides fragilis is an anaerobic commensal constituting only 1-2% of the micro-flora of the human gastrointestinal tract, yet it is the predominant anaerobic isolate in cases of intraabdominal sepsis and bacteremia. B. fragilis can play two roles in the host: in its role as friendly commensal, it must be able to establish itself in the host intestinal mucosa, to utilize and process polysaccharides for use by the host, and to resist the noxious effects of bile salts. In its role as pathogen, it must be able to attach itself to the site of infection, evade killing mechanisms by host defense, withstand antimicrobial treatment and produce factors that damage host tissue. The cell envelope of B. fragilis, likewise, must be able to function in the roles of aggressor, defender and strategist in allowing the organism to establish itself in the host--whether as friend or foe. Recent studies of the genomes and proteomes of the genus Bacteroides suggest that these organisms have evolved strategies to survive and dominate in the overcrowded gastrointestinal neighborhood. Analysis of the proteomes of B. fragilis and Bacteroides thetaiotaomicron demonstrates both a tremendous capacity to use a wide range of dietary polysaccharides, and the capacity to create variable surface antigenicities by multiple DNA inversion systems. The latter characteristic is particularly pronounced in the species B. fragilis, which is more frequently found at the mucosal surface (i.e., often the site of attack by host defenses). The B. fragilis cell envelope undergoes major protein expression and ultrastructural changes in response to stressors such as bile or antimicrobial agents. These agents may also act as signals for attachment and colonization. Thus the bacterium manages its surface characteristics to enable it to bind to its target, to use the available nutrients, and to avoid or evade hostile forces (host-derived or external) in its multiple roles.