Direct analysis of thermophilic bacteria in sewage sludge compost

To analyze metabolically active thermophilic bacteria, a high-temperature direct viable count (HT-DVC) method was developed and applied to sewage sludge compost made by a hyperthermal composting method. When the HT-DVC method was conducted at 60°C and 80°C, maximum numbers of 23.3×10⁸ and 2.62×10⁸ cells/(g of dry sample) of elongated cells (length > 4μm), respectively, were detected. These results indicate that the HT-DVC method can be used to enumerate even metabolically active extreme thermopniles. Strain TH, a Gram-negative, spore-forming, and extremely thermophilic bacterium, which showed growth at 55-78°C, was isolated from the sewage sludge compost. Strain TH is closely related to Caldaterra satsumae YM081. The HT-DVC method could detect strain TH inoculated into sewage sludge compost with autoclaving, but could not selectively detect the strain inoculated into the compost without autoclaving.