Bacteroides fragilis is a common bacterium found within the human body. This microorganism exhibits a complex nature, acting as both a beneficial partner in maintaining health and, under certain conditions, as a source of serious infections.
Understanding B. fragilis
Bacteroides fragilis is a Gram-negative, rod-shaped bacterium that thrives in oxygen-limited environments. It is a prominent member of the Bacteroides group, residing primarily in the colon, where B. fragilis can account for up to 20% of the total microbial population in this region.
This bacterium is non-spore-forming and displays resilience within the fluctuating conditions of the gastrointestinal tract. Its presence is considered a normal component of the colon’s microbial community, contributing to various physiological processes. The high prevalence of B. fragilis highlights its established role in the gut’s complex bacterial ecosystem.
Beneficial Contributions of B. fragilis
B. fragilis promotes gut health by breaking down complex carbohydrates that human enzymes cannot digest. Through fermentation, it produces short-chain fatty acids (SCFAs), such as acetate, propionate, and butyrate. Butyrate serves as a primary energy source for the cells lining the colon, supporting intestinal health. These SCFAs also contribute to maintaining the integrity of the intestinal barrier.
Beyond its metabolic contributions, B. fragilis also modulates the immune system. It produces Polysaccharide A (PSA), which stimulates the development of regulatory T cells (Tregs). These T cells are crucial for maintaining immune tolerance and preventing excessive inflammatory responses. PSA can protect against inflammation by influencing specific immune pathways.
When B. fragilis Causes Harm
Despite its beneficial roles, B. fragilis can become an opportunistic pathogen when the gut barrier is compromised. This often occurs due to trauma, surgery, or underlying diseases that disrupt the mucosal lining of the colon. Once displaced from the gut into sterile body sites, such as the abdominal cavity or bloodstream, B. fragilis can cause severe infections.
It is the most common anaerobic bacterium isolated in human infections, particularly intra-abdominal abscesses and peritonitis. These infections are often polymicrobial. B. fragilis possesses virulence factors, including a polysaccharide capsule, which helps it evade immune defenses and promotes the formation of abscesses, creating an oxygen-free environment where it can thrive. Certain strains of B. fragilis can produce a toxin linked to inflammatory diarrhea and potentially colorectal cancer. A challenge in treating these infections is its high resistance to many common antibiotics, including penicillins, due to beta-lactamase enzymes.
Therapeutic Applications
The immunomodulatory properties of B. fragilis, its Polysaccharide A (PSA), are being investigated for therapeutic uses. PSA has shown promise in animal models for conditions like inflammatory bowel disease (IBD) and multiple sclerosis, helping regulate immune responses and reduce inflammation. It achieves this by promoting the conversion of naive T cells into regulatory T cells, controlling autoimmune responses.
The potential of B. fragilis in neurodevelopmental disorders is also being explored. For example, studies indicate certain strains of B. fragilis may improve behavioral and gastrointestinal symptoms in children with autism spectrum disorder. These investigations aim to develop live biotherapeutic products or specific bacterial components, like PSA, to treat various immune-mediated and neurological conditions.