f Isolation and Characterization of Methanogenium aggregans sp. nov.

A new mesophilic methanogenic bacterium was isolated from a sewage sludge digestor. The cells were irregular cocci (diameter, 0.5 to 2.0 μm) and occurred singly or in small aggregates. Adjacent cells did not appear to touch, even in aggregates. This bacterium was not motile and did not possess flagella. Colonies appearing after 1 week of incubation were white and 1 mm in diameter. Older colonies were yellow and up to 4 mm in diameter. The organism used H2-CO2 or formate as an energy source. Acetate and either yeast extract or Trypticase peptone were required for growth. The optimum pH for growth was between 6.5 and 7.0. The NaCI requirement for methanogenesis was less than 0.2 g/liter. The deoxyribonucleic acid base composition was 52 mol% guanine plus cytosine. This isolate is named Methanogenium aggregans sp. nov., and the type strain (strain MSt) has been deposited in the Deutsche Sammlung von Mikroorganismen as strain DSM 3027.