OBLIGATELY AND FACULTATIVELY AUTOTROPHIC, SULFUR-OXIDIZING 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-f orming strains were gram-negative rods growing at 60-80 degrees C, wit h an optimum at 70-75 degrees C, but only under microaerophilic condit ions (5 kPa oxygen). These strains had similar DNA G+C content (34.7-3 7.6 mol%) and showed a high DNA:DNA homology (70-87%) with Hydrogenoba cter strains isolated from geothermal areas. The facultatively autotro phic strains isolated from hot composts were gram-variable rods that f ormed spherical and terminal endospores, except for one strain. The st rains grew at 55-75 degrees C, with an optimum at 65-70 degrees C. The se bacteria were able to grow heterotrophically, or autotrophically wi th hydrogen; however, they oxidized thiosulfate under mixotrophic grow th conditions (e.g. pyruvate or hydrogen plus thiosulfate). These stra ins had similar DNA G+C content (60-64 mol%) to and high DNA:DNA homol ogy (> 75%) with the reference strain of Bacillus schlegelii. This is the first report of thermogenic composts as habitats of thermophilic s ulfur- 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 degree s C support only a very low diversity of obligatory heterotrophic ther mophiles related to Bacillus stearothermophilus.