A methane-utilizing organism capable of growth both on methane and on more complex organic substrates as a sole source of carbon and energy, has been isolated and studied in detail. Suspensions of methane-grown cells of this organism oxidized C-1 compounds (methane, methanol, formaldehyde, formate); hydrocarbons (ethane, propane); primary alcohols (ethanol, propanol); primary aldehydes (acetaldehyde, propionaldehyde); alkenes (ethylene, propylene); dimethylether; and organic acids (acetate, malate, succinate, isocitrate). Suspensions of methanolor succinate-grown cells did not oxidize methane, ethane, propane, ethylene, propylene, or dimethylether, suggesting that the enzymatic systems required for oxidation of these substrates are induced only during growth on methane. Extracts of methane-grown cells contained a particulate reduced nicotinamide adenine dinucleotide-dependent methane monooxygenase activity. Oxidation of methanol, formaldehyde, and primary alcohols was catalyzed by a phenazine methosulfatelinked, ammonium ion-requiring methanol dehydrogenase. Oxidation of primary aldehydes was catalyzed by a phenazine methosulfate-linked, ammonium ionindependent aldehyde dehydrogenase. Formate was oxidized by a nicotinamide adenine dinucleotide-specific formate dehydrogenase. Extracts of methane-grown, but not succinate-grown, cells contained the key enzymes of the serine pathway, hydroxypyruvate reductase and malate lyase, indicating that the enzymes of C-1 assimilation are induced only during growth on C-1 compounds. Glucose-6-phosphate dehydrogenase was induced during growth on glucose. Extracts of methanegrown cells contained low levels of enzymes of the tricarboxylic acid cycle, including a-keto glutarate dehydrogenase, relative to the levels found during growth on succinate.Among the methylotrophic organisms are both obligate and facultative methylotrophs. Obligate methylotrophs are capable of utilizing only carbon compounds containing no carboncarbon bonds (methane, methanol, dimethylether, methylamine, etc.) as sole sources of carbon and energy. Facultative methylotrophs can use compounds containing no carbon-carbon bonds as well as complex organic compounds with carbon-carbon bonds as sole sources of carbon and energy (6).Almost all methane-utilizing bacteria isolated in pure culture exhibit obligate growth requirement for methane, methanol, or dimethylether as a source of carbon and energy (5,9,11,19,32,35). Organic compounds were unable to serve as a carbon and energy source. Recently, Patt et al. (25) reported the isolation and characterization of bacteria in pure culture that utilize methane as well as the complex organic compounds as sources of carbon and energy.In this paper we report on the isolation of both obligate and facultative methane-utilizing bacteria and on the characterization of one of the facultative isolates.MATERIALS AND METHODS Bacterial strain. Methylobacterium organophilum strain xx (ATCC 27886) (27) was obtained from the American Type Culture Collection, Rockvile, Md. Media a...