Bacteroidetes are a major phylum of bacteria and one of the most abundant members of the human gut microbiome. These gram-negative bacteria are particularly prevalent in the colon, and their population varies based on geography, age, and diet. Among the diverse genera within this phylum, Bacteroides is one of the most significant and well-studied.
Primary Functions of Bacteroidetes
The primary function of Bacteroidetes in the human gut is metabolic, involving the breakdown of complex carbohydrates and dietary fibers that human enzymes cannot digest. These bacteria possess a wide array of enzymes that allow them to ferment these resistant starches and polysaccharides, producing beneficial compounds for their host.
Through this breakdown, Bacteroidetes generate short-chain fatty acids (SCFAs), mainly butyrate, propionate, and acetate. These molecules are an energy source for the cells lining the colon (colonocytes) and play a part in maintaining the health and integrity of the gut lining. Butyrate, in particular, is thought to have antineoplastic properties.
These SCFAs are absorbed into the bloodstream and can influence metabolic processes throughout the body, including lipid metabolism and insulin sensitivity. Some strains of Bacteroidetes also contribute to the synthesis of vitamins B and K.
The Bacteroidetes to Firmicutes Ratio and Health
The gut microbiome is dominated by two major bacterial phyla: Bacteroidetes and Firmicutes. The balance between these groups, expressed as the Bacteroidetes to Firmicutes (B/F) ratio, has become a focus of scientific research. This ratio is viewed as an indicator of gut health and has been linked to various health conditions.
Alterations in the B/F ratio are frequently associated with metabolic health, particularly body weight. A higher proportion of Bacteroidetes relative to Firmicutes is often associated with a leaner body mass. Conversely, a lower B/F ratio has been linked with obesity, as Bacteroidetes are generally less efficient at extracting calories from food compared to Firmicutes.
This link is a correlation, not a direct cause-and-effect relationship, and the connection is the subject of ongoing investigation. The B/F ratio is not static and can change with age, diet, and lifestyle, suggesting that it is a dynamic measure of gut health.
Impact on the Immune System and Disease
Bacteroidetes play a significant part in modulating the host’s immune system, helping develop and maintain a balanced immune response. This interaction “trains” the immune system to distinguish between beneficial microbes and dangerous pathogens. This promotes immune tolerance and helps prevent inappropriate inflammatory reactions within the gut.
A specific species, Bacteroides fragilis, has been studied for its role in promoting T-cell responses, a component of the adaptive immune system. By contributing to a well-regulated immune environment, Bacteroidetes help limit the colonization of pathogenic bacteria through direct competition for nutrients and by fostering a robust immune presence.
An imbalance in the gut microbial community, known as dysbiosis, can affect the abundance of Bacteroidetes. Shifts in their levels have been associated with health conditions like inflammatory bowel disease (IBD). Some species can act as opportunistic pathogens if the intestinal barrier is compromised, leading to infections.
How Diet Shapes Bacteroidetes Populations
The composition of the gut microbiota, including the abundance of Bacteroidetes, is significantly influenced by diet. Dietary choices can shape the gut environment, favoring the growth of certain bacterial populations over others.
Foods high in complex carbohydrates and dietary fiber are particularly effective at supporting Bacteroidetes growth. These foods provide the indigestible polysaccharides that Bacteroidetes use as their primary fuel source. These include plant-based foods such as:
- Fruits
- Vegetables
- Legumes
- Whole grains
In addition to fiber, compounds known as polyphenols, found in foods like berries, dark chocolate, and tea, can also influence the gut microbiome. A diet rich in these plant-derived nutrients helps to sustain a robust and diverse Bacteroidetes population.