Various surface structures can be expressed in Bacteroides fragilis, but little is known about capsular structures in other non-spore-forming anaerobes. Fimbriae have been isolated from Bacteroides fragilis and Porphyromonas gingivalis. The importance of iron-repressible outer membrane proteins as virulence factors in Bacteroides fragilis is under study. The low endotoxic activity of Bacteroides fragilis lipopolysaccharide can be attributed to the chemical composition of this organism's lipid A. A tissue culture system for the demonstration of Bacteroides fragilis enterotoxin has recently been described. The toxins A and B of Clostridium difficile are immunologically distinct. The importance of IgA proteases and other enzymes as virulence factors in anaerobic bacteria remains unclear.