The in vivo formation of methane and of several S-methyl volatile compounds from the terminal S-methyl group of l-methionine is reported for growing cultures of four Clostridium strains (C. hastiforme, C. histolyticum, C. subterminale, and Clostridium sp. strain DSM 1786). After growth in 5 ml of unamended medium, C. hastiforme formed the highest amount of methane (408 nmol per tube in the headspace). When the culture medium was amended with 100 mM l-[S-methyl-2H3]methionine, the four strains formed [2H3]methane (proportion in the methane peak, >85%) as well as methanethiol, dimethyl disulfide, dimethyl trisulfide, and S-methyl thioacetate labeled on the methyl moiety. Methanethiol is also a precursor of methane for Clostridium sp. strain DSM 1786. The trace methane formation observed for these four proteolytic, nonglucidolytic Clostridium strains can be of ecological interest, particularly in aquatic sediments and in the gastrointestinal tract of humans and animals. It can explain in part the trace methane formation which cannot be ascribed to methanogens sensu stricto.
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