The human body is home to trillions of microorganisms, collectively known as the gut microbiome, which functions as a complex, highly active metabolic organ profoundly influencing host health. Among the thousands of species inhabiting the lower gastrointestinal tract, the genus Bacteroides stands out as one of the most abundant and functionally important groups. Exploring the biology of Bacteroides reveals their central involvement in nutrient utilization and the proper development of the host immune system.
Defining the Bacteroides Group
The bacteria belonging to the genus Bacteroides are Gram-negative, rod-shaped obligate anaerobes, meaning they cannot survive in the presence of oxygen. They thrive in the oxygen-poor environment of the human colon, reaching concentrations up to \(10^{11}\) cells per gram of feces, making them a substantial fraction of the total microbial population. Bacteroides is the namesake genus of the phylum Bacteroidetes, one of the four most prevalent phyla found in the adult human gut. The genus contains numerous species, with two extensively studied model species being Bacteroides fragilis and Bacteroides thetaiotaomicron. While B. fragilis is known for its potent immunomodulatory effects, B. thetaiotaomicron is recognized for its vast metabolic capabilities. Their metabolic flexibility and abundance establish them as a dominating force in the gut.
Essential Roles in Nutrient Processing
The primary function of Bacteroides is to break down complex carbohydrates, such as dietary fiber and starches, that the human body cannot digest. These undigested molecules pass into the colon. The Bacteroides genus possesses Polysaccharide Utilization Loci (PULs), a large set of genes encoding the enzymes necessary to dismantle these molecules. This microbial degradation, known as fermentation, creates metabolic byproducts called Short-Chain Fatty Acids (SCFAs).
The three most abundant SCFAs are acetate, propionate, and butyrate. These small molecules are readily absorbed by the host and serve as a significant energy source. Butyrate is the preferred energy source for colonocytes, the epithelial cells lining the colon. Supplying energy to these cells helps maintain the integrity of the gut barrier, reducing intestinal permeability. Acetate and propionate travel through the bloodstream, influencing glucose homeostasis and lipid metabolism in the liver and other tissues.
Shaping Immune System Development
Bacteroides play a profound role in shaping and training the host immune system, particularly in early life. Colonization is associated with the normal maturation of immune tissues and the development of balanced immune responses. A well-studied example is Polysaccharide A (PSA), a molecule found on the surface of B. fragilis. PSA is processed by Antigen-Presenting Cells (APCs) and presented to CD4+ T-cells via MHC-II molecules. This activation promotes the generation of regulatory T-cells (Tregs), specialized immune cells that suppress excessive inflammation.
Tregs produce anti-inflammatory signaling molecules, such as Interleukin-10 (IL-10), which help maintain immune tolerance and prevent overreactions to harmless substances. Bacteroides also contribute to colonization resistance by preventing pathogenic bacteria from establishing themselves in the gut. For instance, B. thetaiotaomicron suppresses toxin release from Clostridium difficile.
Influencing Bacteroides Levels Through Lifestyle
The abundance of Bacteroides is highly responsive to lifestyle factors, with diet being the most influential modulator. A high-fiber diet, rich in plant-based foods, provides the complex carbohydrates that Bacteroides ferment. Prebiotic fibers, such as inulin, are selectively fermented by certain Bacteroides species, promoting their growth. Conversely, a prolonged diet high in animal fats and low in fiber is associated with different Bacteroides profiles and can negatively impact the gut ecosystem.
Antibiotics represent a significant disruptor, indiscriminately reducing the total microbial population, including beneficial Bacteroides. The resulting imbalance compromises colonization resistance and increases susceptibility to pathogens. Restoration strategies involve dietary interventions and the use of targeted probiotics or synbiotics, which combine prebiotics with beneficial bacteria.