Methanobrevibacter oralis is a species of methanogenic archaea that inhabits the human oral cavity, specifically the gums. While research in 2004 initially suggested a causal relationship between the activity of these methanogens and periodontal disease, subsequent studies demonstrated that no such causal link exists. Instead, periodontal disease likely causes an enlargement of anaerobic regions in the mouth, which are subsequently colonized by M. oralis.