Digestive tract methanodrome: Physiological roles of human microbiota-associated methanogens

Methanogens are the archaea most commonly found in humans, in particular in the digestive tract and are an integral part of the digestive microbiota. They are present in humans from the earliest moments of life and represent the only known source of methane production to date. They are notably detected in humans by microscopy, fluorescent in situ hybridization, molecular biology including PCR-sequencing, metagenomics, matrix-assisted laser desorption ionization time-of-flight mass spectrometry and culture. Methanogens present in the human digestive tract play major roles, in particular the use of hydrogen from the fermentation products of bacteria, thus promoting digestion. They are also involved in the transformation of heavy metals and in the use of trimethylamine produced by intestinal bacteria, thus preventing major health problems, in particular cardiovascular diseases. Several pieces of evidence suggest their close physical contacts with bacteria support symbiotic metabolism. Their imbalance during dysbiosis is associated with many pathologies in humans, particularly digestive tract diseases such as Crohn's disease, ulcerative colitis, diverticulosis, inflammatory bowel disease, irritable bowel syndrome, colonic polyposis, and colorectal cancer. There is a huge deficit of knowledge and partially contradictory information concerning human methanogens, so much remains to be done to fully understand their physiological role in humans. It is necessary to develop new methods for the identification and culture of methanogens from clinical samples. This will permit to isolate new methanogens species as well as their phenotypic characterization, to explore their genome by sequencing and to study the population dynamics of methanogens by specifying in particular their exact role within the complex flora associated with the mucous microbiota of human.