The human gut is a complex ecosystem, home to trillions of microorganisms collectively known as the gut microbiome. While most research focuses on Bacteria, the microbial domain Archaea also plays a significant role in gut function. Methanospirillum hungatei is a specific archaeon that inhabits the digestive tract of some humans. As a methanogen, this microbe produces methane gas as a byproduct of its metabolism. Studying M. hungatei is important because its activity influences the overall metabolic balance of the gut, impacting host health and specific gastrointestinal conditions.
Classification and Unique Biology
Methanospirillum hungatei is classified within the domain Archaea, placing it in a separate kingdom of life from Bacteria and Eukaryotes. It belongs to the phylum Euryarchaeota and the order Methanomicrobiales. This classification highlights its unique evolutionary history and distinct cellular machinery compared to the bacteria that dominate the gut environment.
The microbe is morphologically distinctive, characterized as a narrow, curved rod, or “spirillum.” Individual cells measure approximately 0.5 micrometers in diameter and 7 micrometers in length. These cells are often encased within a proteinaceous sheath-like structure, which can contain multiple cells and form long chains extending up to 100 micrometers.
Surrounding the cell membrane is a specialized, proteinaceous surface layer (S-layer). Like many organisms in the digestive tract, M. hungatei is a strictly obligate anaerobe, meaning it cannot survive in the presence of oxygen.
The Specific Role: Methanogenesis
The primary function of Methanospirillum hungatei is the production of methane (\(\text{CH}_4\)), a process known as methanogenesis. This microbe is a hydrogenotrophic methanogen, meaning it generates energy by using hydrogen gas (\(\text{H}_2\)) to reduce carbon dioxide (\(\text{CO}_2\)). The microbe can also use formate, a simple organic acid, as a substrate for this energy-generating process. The process relies on a complex series of enzymatic reactions that utilize specialized cofactors like coenzyme M and coenzyme B.
The consumption of hydrogen gas allows M. hungatei to function as a “hydrogen sink” within the gut ecosystem. This activity is crucial for syntrophy, a symbiotic relationship it forms with other gut bacteria. Fermenting bacteria produce hydrogen gas as a metabolic waste product; if this hydrogen accumulates, it inhibits their ability to continue fermentation and extract energy from food. By constantly consuming the hydrogen, M. hungatei keeps the partial pressure of \(\text{H}_2\) low, making the fermentation process thermodynamically favorable for its bacterial partners.
Ecological Impact and Clinical Associations
The methane gas produced by M. hungatei and other methanogens has consequences for gastrointestinal function and overall health. A strong association exists between the presence of methane in the gut and certain gastrointestinal disorders, particularly constipation-predominant Irritable Bowel Syndrome (C-IBS). Methane gas affects the muscle contractions of the digestive tract, potentially by acting as a signaling molecule. This action slows down the movement of contents through the small intestine and colon, known as reduced gut motility. Studies show that individuals who produce detectable levels of methane gas often report greater severity of constipation, and the quantity of methane measured is proportional to symptom severity. This observation led to the concept of Intestinal Methanogen Overgrowth (IMO), which is linked to chronic, slow-transit constipation.
Beyond motility issues, methanogen function is hypothesized to affect host metabolism and weight regulation. Individuals who test positive for methane production tend to have a significantly higher Body Mass Index (BMI) and greater percentage of body fat. The proposed mechanism connects to the methanogen’s role as a hydrogen sink. By efficiently removing hydrogen, M. hungatei enhances the ability of its bacterial partners to ferment food, allowing them to extract more energy and calories. This increased efficiency in caloric harvest is thought to contribute to the association between methanogen presence and increased body weight.
Methods to alter the balance of the gut community include specific dietary changes that reduce the availability of substrates for fermentation, which limits the hydrogen available to methanogens. Targeted interventions, such as certain antibiotics, are also used to reduce the population of methane-producing archaea in the gut. These approaches aim to shift the microbial balance to alleviate symptoms associated with methane production.