Heat Resistance Correlated with DNA Content in Bacillus megaterium Spores

Two subpopulations of Bacillus megaterium spores (1.360 and 1.355 g/ml) were obtained by density gradient centrifugation. The heavier spores had a higher thermoresistance (e.g., D(80) = 186 versus 81 min) and a higher DNA content (1.25 x 10 versus 0.65 x 10 g per spore, apparently corresponding to digenomic versus monogenomic spores). No appreciable differences were found in the mineral and dipicolinic acid contents or in the inactivation kinetics of the two subpopulations. The implications of the findings are discussed with regard to mechanisms of heat resistance and of inactivation.