Bacterial growth with chlorinated methanes.

Chlorinated methanes are important industrial chemicals and significant environmental pollutants. While the highly chlorinated methanes, trichloromethane and tetrachloromethane, are not productively metabolized by bacteria, chloromethane and dichloromethane are used by both aerobic and anaerobic methylotrophic bacteria as carbon and energy sources. Some of the dehalogenation reactions involved in the utilization of the latter two compounds have been elucidated. In a strictly anaerobic acetogenic bacterium growing with chloromethane, an inducible enzyme forming methyltetrahydrofolate and chloride from chloromethane and tetrahydrofolate catalyzes dehalogenation of the growth substrate. A different mechanism for the nucleophilic displacement of chloride is observed in aerobic methylotrophic bacteria utilizing dichloromethane as the sole carbon and energy source. These organisms possess the enzyme dichloromethane dehalogenase which, in a glutathione-dependent reaction, converts dichloromethane to inorganic chloride and formaldehyde, a central metabolite of methylotrophic growth. Sequence comparisons have shown that bacterial dichloromethane dehalogenases belong to the glutathione S-transferase enzyme family, and within this family to class Theta. The dehalogenation reactions underlying aerobic utilization of chloromethane by a pure culture and anaerobic growth with dichloromethane by an acetogenic mixed culture are not known. It appears that they are based on mechanisms other than nucleophilic attack by tetrahydrofolate or glutathione.