The human body is home to a vast and complex community of microorganisms known as the microbiome, which plays a profound role in host health. This microbial ecosystem, located primarily in the digestive tract, contains trillions of bacteria that perform functions the human body cannot execute alone. Among the most abundant and functionally important members of this community is the phylum Bacteroidetes, a major group of bacteria residing predominantly in the gut.
Defining the Bacteroidetes Phylum
Bacteroidetes is a phylum that includes thousands of different species. These bacteria share general characteristics, being Gram-negative and predominantly anaerobic, meaning they thrive in the oxygen-deprived environment of the intestine. They are typically rod-shaped organisms that colonize the large intestine, where most microbial activity related to nutrient salvage occurs.
The phylum contains several well-studied genera, most notably Bacteroides and Prevotella. The relative abundance of these genera is heavily influenced by the host’s diet. For instance, Bacteroides species tend to dominate in individuals consuming a Western-style diet high in animal protein and fat. Conversely, Prevotella species are more common in people whose diets are rich in plant matter and complex carbohydrates.
Essential Role in Fiber Digestion
The primary function of Bacteroidetes is the breakdown of complex carbohydrates that human digestive enzymes cannot process. These bacteria possess specialized genetic clusters called Polysaccharide Utilization Loci (PULs), which encode the necessary enzymes to dismantle these large, non-digestible molecules. This metabolic action allows the host to salvage calories and nutrients that would otherwise be excreted.
The fermentation of these complex carbohydrates yields valuable byproducts known as Short-Chain Fatty Acids (SCFAs), with acetate, propionate, and butyrate being the most abundant. Butyrate, in particular, serves as the preferred energy source for the colonocytes, helping to maintain the integrity of the gut barrier. Propionate and acetate travel through the bloodstream to the liver and other tissues, where they are utilized in processes like glucose and lipid metabolism.
The Balance with Firmicutes
Bacteroidetes and Firmicutes are the two most dominant bacterial phyla in the human gut, often accounting for over 90% of the microbial community. The relative proportion of these two groups, expressed as the Firmicutes-to-Bacteroidetes (F/B) ratio, is a frequently studied metric in microbiome research. This ratio is considered a simplified indicator of the overall gut ecosystem’s state and is highly sensitive to changes in diet.
A high F/B ratio, indicating a lower proportion of Bacteroidetes, has been consistently associated with conditions like obesity and metabolic disorders. Conversely, a low F/B ratio, suggesting a relative abundance of Bacteroidetes, has sometimes been observed in cases of Inflammatory Bowel Disease (IBD). This ratio reflects the ecological balance and functional capacity of the gut community. Dietary interventions, such as increasing fiber intake, can rapidly shift this ratio by favoring the growth of Bacteroidetes.
Impact on Metabolic and Immune Health
The metabolic activities of Bacteroidetes have profound systemic consequences, particularly for host metabolic and immune health. The SCFAs they produce are known to modulate appetite regulation and glucose homeostasis. Low Bacteroidetes abundance and a resulting decrease in SCFA production are frequently observed in individuals with Type 2 diabetes and obesity, suggesting a link between the microbial community’s function and these metabolic conditions.
Beyond metabolism, Bacteroidetes are deeply involved in regulating the immune system. Species like Bacteroides fragilis have been shown to interact with the host’s immune cells to promote immunotolerance and reduce inflammation. SCFAs support the integrity of the intestinal barrier, which prevents the leakage of bacterial products into the bloodstream. The presence of a healthy Bacteroidetes population is therefore linked to a reduced risk of inflammatory diseases and helps in the proper development of the immune system throughout life.