Two strains of Moorella thermoacetica, JW/B-2 and JW/DB-4, isolated as contaminants from autoclaved media for chemolithoautotrophic growth containing 0.1% (wt/vol) yeast extract, formed unusually heat-resistant spores. Spores of the two strains required heat activation at 100 °C of more than 2 min and up to 90 min for maximal percentage of germination. Kinetic analysis indicated the presence of two distinct subpopulations of heat-resistant spores. The decimal reduction time (D₁₀-time=time of exposure to reduce viable spore counts by 90%) at 121°C was determined for each strain using spores obtained under different conditions. For strains JW/DB-2 and JW/ DB-4, respectively, spores obtained at ~25 °C from cells grown chemolithoautotrophically had D₁₀-times of 43 min and 23 min; spores obtained at 60 °C from cells grown chemoorganoheterotrophically had D10-times of 44 min and 38 min; spores obtained at 60 °C from cells grown chemolithoautotrophically had D₁₀-times of 83 min and 111 min. The thickness of the cortex varied between 0.10 and 0.29 µm and the radius of the cytoplasm from 0.14 to 0.46 µm. These spores are amongst the most heat-resistant noted to date. Electron microscopy revealed structures within the exosporia of spores prior to full maturity that were assumed to be layers of the outer spore coat.
Keywords: Heat-resistance, Endospores, Sporulation, D10-time, Moorella thermoacetica, Spore ultrastructure