None of the other Archaea are pathogens either, but considering the conditions under which they grow, this is not surprizing. Methanogens associate with these organisms, utilizing the hydrogen they generate for methanogenesis, and keep the hydrogen concentrartion low enough for the heterotrophs to make a living. Neither of these organisms could persist in the environment alone, but together they are successful. Other than the fact that they all make a living the same way, methanogens are a diverse phenotypic and ecological group. These are marine and freshwater sediment irregular cocci.
Some species are thermophiles, but many mesophilic species have been isolated as well.
They are motile via tufted flagella, but they are so sensitive to oxygen that motility can only be observed if samples are taken and observed microscopically in a strictly anaerobic environment. Their cell walls are made up of extracellular protein. Some are complete prototrophs - can make everything they need from inorganic compounds. They are mostly thermophiles, and are more easily isolated that other methanogens because they are more resistant to exposure to oxygen. Mesophilic or moderately thermophilic species are common colon and rumen inhabitants in animals.