Selective enrichment and purification of cultures of Methanosaeta spp

A simple method for the isolation of axenic cultures of members of the obligately acetotrophic methanogenic genus Methanosaeta is described. To overcome the competitive advantage obtained by faster growing acetate-utilizing Methanosarcina spp. in batch enrichment cultures, acetone and isopropanol are used as the growth substrates for the enrichment step. Acetone- and isopropanol-utilizing bacteria slowly ferment these substrates to acetate, which allows Methanosaeta spp. to maintain the acetate concentration at levels below the threshold required for growth of Methanosarcina spp. These enrichments eventually develop dense populations of Methanosaeta spp., which can then be separated from contaminating microorganisms to yield axenic cultures.