Feces, often referred to as stool, represent the solid or semi-solid waste material discharged from the large intestine. Feces are primarily composed of about 75 percent water, with the remaining 25 percent consisting of solid matter. A considerable portion of this solid component, ranging from 25 to 54 percent of the dry mass, is made up of bacterial biomass. Far from being mere waste, feces contain a vast and diverse community of microorganisms, predominantly bacteria, which are a natural and healthy aspect of human biology. These bacteria originate from the human gut microbiota, which encompasses an estimated 10^13 to 10^14 microbial cells, comprising hundreds of different species.
Common Bacteria Found
The large intestine hosts the body’s largest and most densely populated microbial ecosystems. The dominant bacterial groups, known as phyla, commonly found in human feces include Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria. These four phyla constitute the vast majority of the gut microbiota, with Firmicutes and Bacteroidetes alone representing approximately 90 percent of this microbial community.
Specific bacterial genera consistently observed include:
Bacteroides
Clostridium
Faecalibacterium
Eubacterium
Ruminococcus
Peptococcus
Peptostreptococcus
Bifidobacterium
Less abundant but regular inhabitants include Escherichia (e.g., E. coli) and Lactobacillus. A notable species, Faecalibacterium prausnitzii (Firmicutes), is frequently identified as one of the most common bacteria in healthy adults. Bacteria in the large intestine and feces are largely obligate anaerobes, thriving in low-oxygen environments; Bacteroides and Bifidobacterium are prime examples. Bifidobacterium species are particularly prevalent in infants, especially breast-fed ones, though their dominance decreases with age.
Roles of These Microbes
These microbial communities perform many functions that contribute to human health. A significant role involves the digestion of complex carbohydrates and dietary fibers, which human enzymes cannot break down. Through fermentation, these bacteria produce short-chain fatty acids (SCFAs), such as acetic, propionic, and butyric acid. These SCFAs are absorbed by the host, providing an important energy source for gut cells and other bodily processes. Butyrate, in particular, serves as a primary energy source for colon cells and possesses properties that support gut health.
Beyond digestion, gut bacteria synthesize various essential vitamins. This includes vitamin K and a range of B vitamins (B1, B2, B3, B5, B6, B7, B9, and B12). Certain Bacteroides species, P. copri, and E. rectale contribute substantially to B-vitamin synthesis. Specific bacteria also convert vitamin K1 to its active form, K2, which supports bone health and cardiovascular function.
The gut microbiota also develops and regulates the immune system. Microbes interact with immune cells, influencing their function and maintaining the integrity of the gut barrier. This interaction is important for proper immune modulation and for establishing tolerance to beneficial microbes while defending against harmful ones.
Factors Shaping Fecal Microbes
The composition and diversity of bacteria in an individual’s feces are influenced by internal and external factors. Diet is a significant determinant, as the diversity of plant fibers consumed directly impacts microbial diversity. Diets rich in fiber promote the growth of beneficial bacteria and the production of beneficial short-chain fatty acids. Conversely, dietary patterns high in sugar, saturated fats, or processed foods can lead to imbalances in the microbial community. The gut microbiota can respond rapidly to dietary changes, with shifts in composition occurring even within a single day.
Age also contributes to variations in fecal microbial profiles. Infant gut microbiota, initially dominated by Bifidobacterium, matures to an adult composition by approximately three years of age. Microbial diversity increases through childhood and stabilizes in adulthood. Lifestyle factors like exercise and stress, and geographic location, also influence these communities. Medication use, particularly antibiotics, can significantly disrupt the microbial population by reducing overall diversity.
Insights from Fecal Bacteria
Analysis of fecal bacteria offers valuable insights into an individual’s health status. Imbalances or significant alterations in these bacterial communities, referred to as dysbiosis, are associated with various health conditions. Dysbiosis has been linked to intestinal disorders such as inflammatory bowel disease (IBD) and irritable bowel syndrome.
Beyond the digestive tract, shifts in the gut microbiome have been correlated with broader health concerns. These include metabolic disorders like obesity and type 2 diabetes, as well as neurological conditions such as Parkinson’s disease, Alzheimer’s disease, anxiety, and depression. Researchers use advanced techniques like 16S rRNA gene sequencing and metagenomics to study these microbial communities and understand their functions. This ongoing research aims to identify specific microbial patterns associated with health and disease, potentially leading to earlier diagnoses and personalized treatment approaches.