The genus Methanobrevibacter is a common resident of the human and animal gastrointestinal tract. This microorganism is highly prevalent in the lower digestive system, thriving in the oxygen-deprived environment of the colon. Its presence and abundance in the gut microbiome are linked to significant differences in human metabolism and digestive function. Understanding the unique biology and metabolic processes of this organism is a major focus in microbiology and digestive health.
Identifying the Unique Life Form
Methanobrevibacter is classified into the domain Archaea, a distinct branch of life separate from Bacteria and Eukarya. Although Archaea may appear similar to bacteria, they possess fundamental genetic and structural differences. One distinction lies in the cell wall composition, as archaeal cells do not contain peptidoglycan, the rigid polymer found in bacterial cell walls.
Instead, Methanobrevibacter utilizes a chemically distinct compound called pseudopeptidoglycan, or a protective layer composed of surface-layer proteins. Further differences include the cell membrane lipids, which are ether-linked in Archaea rather than the ester-linked fatty acids found in Bacteria. This organism is also defined as a methanogen, a microbe whose metabolism results in the production of methane gas.
The Primary Role in Gut Metabolism
The function of Methanobrevibacter in the gut ecosystem is its role as a hydrogenotrophic methanogen, generating methane by utilizing hydrogen gas. This organism acts as a “hydrogen sink” within the colon, supporting the entire microbial community. During the breakdown of complex carbohydrates by other gut bacteria, a large amount of metabolic hydrogen ($\text{H}_2$) is produced as a byproduct.
If $\text{H}_2$ were allowed to accumulate, it would create a back-pressure that inhibits other bacteria, slowing the fermentation process of dietary fibers. Methanobrevibacter solves this by scavenging the excess $\text{H}_2$ and combining it with carbon dioxide ($\text{CO}_2$) to yield methane ($\text{CH}_4$) and water. This removal allows primary fermenting bacteria to operate more efficiently, ensuring a continuous breakdown of undigested food material.
This symbiotic relationship benefits the methanogen with a constant supply of $\text{H}_2$ and $\text{CO}_2$, while fermenting bacteria benefit from the removal of their inhibitory waste product. This mechanism can significantly increase the total amount of energy extracted from food. The final product, methane, is largely expelled from the body through breath or flatus.
Connection to Human Health and Disease
The metabolic activity of Methanobrevibacter influences human health, particularly energy balance and gut motility. By efficiently removing hydrogen, the organism promotes faster carbohydrate fermentation by other microbes. This leads to increased production of short-chain fatty acids (SCFAs) like acetate, propionate, and butyrate. These SCFAs are absorbed by the host and represent a significant source of usable calories, contributing up to 10% of daily caloric intake.
Elevated populations of this methanogen are proposed to increase the host’s ability to “harvest” calories from food, potentially playing a role in body weight regulation. Furthermore, the methane gas produced by Methanobrevibacter is strongly associated with functional digestive disorders. High methane levels are linked to the constipation-dominant subtype of Irritable Bowel Syndrome (IBS-C).
Methane gas is believed to act as a neuromodulator, directly impacting the nerves that control digestive tract movement. Studies show that methane can significantly slow intestinal transit time, with some models demonstrating a decrease in small intestinal transit speed by an average of 59%. This slowing effect exacerbates constipation, leading to symptoms like bloating and abdominal discomfort. Methane breath testing is now used as a non-invasive diagnostic tool to identify patients whose constipation is driven by the presence and activity of methanogens.