Methylobacter mesophilicum. Coloured transmission electron micrograph (TEM) of the methanotrophic bacteria Methylobacter mesophilicum. These are Gram-negative bacteria which are able to use methane as a source of energy. The bacteria are environmental, living in soil and plant matter, and do not generally cause infections in humans. If the immune system is weakened, however, they may cause skin ulceration and septicaemia. Magnification unknown.