Bacteroides fragilis is a gram-negative, anaerobic bacterium and a prominent inhabitant of the human gastrointestinal tract, specifically the colon. As one of the best-known members of the gut microbiota, it exists in a symbiotic relationship with its host. This bacterium is uniquely adapted to the oxygen-free environment of the large intestine and plays a part in the normal functioning of the gut ecosystem.
The Commensal Role in Gut Health
Bacteroides fragilis is a key player in digestion, breaking down complex carbohydrates and dietary fibers from plant-based foods that human enzymes cannot process. This fermentation process is beneficial for the human host, as it unlocks nutrients that would otherwise be inaccessible. The breakdown results in the production of short-chain fatty acids (SCFAs), such as butyrate, propionate, and acetate.
Butyrate serves as a primary energy source for the cells lining the colon, known as colonocytes, helping to maintain the integrity of the gut barrier. SCFAs also contribute to a healthy gut environment by lowering the pH of the colon, which inhibits the growth of pathogenic bacteria.
Beyond digestion, B. fragilis helps develop and modulate the host immune system. The bacterium’s presence helps to “train” the immune system to distinguish between harmless commensal bacteria and harmful pathogens. This immune education is mediated by a molecule on its surface called capsular polysaccharide A (PSA). PSA interacts with immune cells, promoting anti-inflammatory molecules and helping to maintain a balanced immune response. By fostering a tolerant immune environment, B. fragilis contributes to the overall stability of the gut microbiome.
When Good Bacteria Turn Pathogenic
Despite its beneficial role, Bacteroides fragilis can become an opportunistic pathogen if it escapes the gut. This transition is not caused by a change in the bacterium, but by its relocation to other parts of the body. The trigger is a breach of the intestinal barrier, allowing the bacterium to enter sterile areas and cause serious infections.
This breach of the intestinal wall can occur for various reasons, including abdominal trauma, a ruptured appendix, or complications from bowel surgery. Conditions such as diverticulitis also create an opportunity for B. fragilis to escape. In these situations, the bacterium finds itself in a new environment where it can thrive and cause disease, triggering an inflammatory response.
Infections Caused by Bacteroides fragilis
After escaping the colon, Bacteroides fragilis can cause various infections, most commonly in the abdomen. These infections often manifest as abscesses—localized collections of pus—in the abdomen, pelvis, or liver. An abscess is the body’s attempt to wall off an infection, but it also serves as a reservoir for bacteria to multiply. These infections are often polymicrobial, meaning they involve a mixture of bacteria, with B. fragilis being a predominant species.
In addition to intra-abdominal abscesses, B. fragilis can cause bacteremia, a bloodstream infection. This occurs when bacteria from an abscess enter the bloodstream, allowing them to travel to other parts of the body. Bacteremia is a serious condition that can lead to sepsis, a life-threatening complication of infection. B. fragilis is also a cause of soft tissue infections, particularly in surgical wounds or diabetic ulcers.
The success of B. fragilis as a pathogen is due to its virulence factors, which are molecules that help it cause disease. Its polysaccharide capsule helps the bacterium evade the host’s immune system by protecting it from being engulfed by immune cells. Some strains also produce Bacteroides fragilis toxin (BFT), which can disrupt the intestinal lining and contribute to inflammation.
Treatment and Antibiotic Resistance
Treatment for Bacteroides fragilis infections involves both surgical intervention and antibiotic therapy. Surgical drainage of abscesses is often necessary to remove the source of infection and improve antibiotic effectiveness. The choice of antibiotic is guided by the bacterium’s natural resistance to many drugs, including penicillin, due to its production of the beta-lactamase enzyme.
Commonly used antibiotics for B. fragilis infections include metronidazole and carbapenems, such as meropenem. While these drugs are effective, the increasing prevalence of antibiotic resistance is a concern. Resistance to metronidazole has been reported, and emerging resistance to carbapenems is also a concern as they are often last-resort antibiotics.
Antibiotic resistance in B. fragilis develops through the acquisition of new genes or mutations in its DNA. This highlights the need for appropriate antibiotic use and ongoing surveillance of resistance patterns. Accurate identification of the organism and its susceptibility is required to select the most effective treatment. The rise of multidrug-resistant strains underscores the need for new therapeutic strategies.