Methanogenic archaea in health and disease: a novel paradigm of microbial pathogenesis.

Microbes that produce methane gas, methanogens, were identified as Archaea in the 1970s but their possible role in disease is only emerging now, after they were found in the large intestine, mouth, and vagina. Significant associations were observed, for instance, between levels of methanogens in periodontal pockets and severity of periodontitis, and between quantities of methanogens in the large intestine and diseases such as colon cancer and diverticulosis. Recently, a role for intestinal methanogens in obesity was proposed. The lesson learned is that for methanogens we have to look at their pathogenicity from a different angle in comparison to classic pathogens that invade tissues and release toxins. This type of pathogenicity has not yet been described for methanogens. Instead, methanogens seem to participate in pathogenicity indirectly, favoring the growth of other microbes, which are directly involved in pathogenesis. This indirect role should not be minimized. On the contrary, it has become clear that a fundamental change of approach to the understanding and control of microbial diseases must be implemented. A comprehensive strategy is needed to elucidate the syntrophic associations that are essential for a healthy relation among microbes (including methanogens) and between them and the host organism, and to unveil those associations that lead to disease.