Obligately and facultatively autotrophic, sulfur- and hydrogen-oxidizing thermophilic bacteria isolated from hot composts

A variety of autotrophic, sulfur- and hydrogen-oxidizing thermophilic bacteria were isolated from thermogenic composts at temperatures of 60-80 degrees C. All were penicillin G sensitive, which proves that they belong to the Bacteria domain. The obligately autotrophic, nonspore-forming strains were gram-negative rods growing at 60-80 degrees C, with an optimum at 70-75 degrees C, but only under microaerophilic conditions (5 kPa oxygen). These strains had similar DNA G+C content (34.7-37.6 mol%) and showed a high DNA:DNA homology (70-87%) with Hydrogenobacter strains isolated from geothermal areas. The facultatively autotrophic strains isolated from hot composts were gram-variable rods that formed spherical and terminal endospores, except for one strain. The strains grew at 55-75 degrees C, with an optimum at 65-70 degrees C. These bacteria were able to grow heterotrophically, or autotrophically with hydrogen; however, they oxidized thiosulfate under mixotrophic growth conditions (e.g. pyruvate or hydrogen plus thiosulfate). These strains had similar DNA G+C content (60-64 mol%) to and high DNA:DNA homology (> 75%) with the reference strain of Bacillus schlegelii. This is the first report of thermogenic composts as habitats of thermophilic sulfur- and hydrogen-oxidizing bacteria, which to date have been known only from geothermal manifestations. This contrasts with the generally held belief that thermogenic composts at temperatures above 60 degrees C support only a very low diversity of obligatory heterotrophic thermophiles related to Bacillus stearothermophilus.