B. fragilis is the most frequent opportunistic anaerobe pathogen isolated from human infections. The success of B. fragilis pathogenicity is still not completely understood. In general, Bacteroides infections occur as a consequence of a disruption in the integrity of the intestinal mucosa wall resulting from conditions such as gastrointestinal surgery, perforated or gangrenous appendicitis, perforated ulcer, diverticulitis, trauma and inflammatory bowel disease.Despite being only 0.5 - 1% of the intestinal microbial flora which typically contains 1011 to 1012 anaerobic bacteria per gram of stool, B. fragilis accounts for 50 to 70 % of all anaerobes isolated from human infections such as, intra-abdominal abscesses, infections of the female genital tract, deep wounds, brain abscesses, bacteremia, and midgestation infectious abortion. Because of this extraordinary paradox, understanding the mechanisms by which B. fragilis break away from the colon to cause systemic disease and death in humans is an extremely important scientific objective.We have found that B. fragilis produces nine hemolysins/cytolysins and their role in B. fragilis pathogenicity is poorly understood. Hemolysins/cytolysins are powerful virulence factors in both Gram positive and Gram negative bacteria. Moreover, bacterial cytolysins/hemolysins play an important role in the equilibrium control of the microbial ecosystem population associated with eukaryotic hosts due to their cytolytic/bactericidal properties which are used to inhibit other prokaryotes and eukaryotes in highly competitive ecological systems. In this regard, current research aims to investigate the role and function of this extensive number of hemolysins/cytolysins in Bacteroides species play in the establishment of infectious process and also their contribution to the ability of Bacteroides to dominate and take over the colonization of the human large intestinal tract.