The Growth Yields and Growth Kinetics of a Thermophilic Bacillus, and the Properties of a Thermophilic Lactate Dehydrogenase

Purpose of the Study: Until recently, organisms which grow at high temperatures have been considered to be unique, and extraordinary mechanisms have been proposed to explain their behavior. This study has attempted to demonstrate that thermophilic bacteria do not display unusual behavior with respect to their growth yields, growth kinetics, and enzyme kinetics. Procedure and Findings: A thermophilic bacterium which closely resembled Bacillus stearothermophilus was isolated from soil near geothermal steam wells. The organism, designated SSC-Tl, grew prototrophically on a variety of simple organic compounds. Its minimum growth temperature was between 42 and 44°c, its optimum was 67 to 68°c, and its maximum was 74. 5°c. Molar growth yields under aerobic conditions in media containing glucose, lactate, or glutamate as the single source of carbon and energy were constant from 49 to 71°c. The magnitudes of the molar growth yields were similar to those usually reported for mesophiles: 44.7 g on glucose, 31.5 g on lactate, and 31.0 g on glutamate. Arrhenius plots of growth rate revealed sharp inflections at the optimum temperature and at 55°c. The temperature coefficient of growth between 55°c and the optimum was 9.1 Kcal/mole in minimal glucose broth and 12.8 Kcal/mole in Trypticase Soy Broth. Temperature coefficients below 55°c and above the optimum were essentially the same in both media, 30 Kcal/mole and 45 Kcal/mole respectively. A heat stable lactate dehydrogenase was partially purified and its substrate kinetics were investigated. Preliminary estimates of Km for pyruvate varied from 1.8 mM at 42°c to 9.9 mM at 64°c. Pyruvate inhibited enzyme activity at high concentrations. Conclusions: A significant portion of the energy metabolism of SSC-T1 is not devoted to the repair of heat damaged macromolecules. Temperature coefficient of growth is not related to growth temperature. The maximum and minimum growth temperatures may be determined by temperature-dependent changes in essential proteins. LDH from SSC-T1 is more heat resistant than LDH from E. coli, but resembles E. coli LDH in its substrate kinetics.