A study of the growth energetics of Bacillus acidocaldarius
A mineral-salts medium was developed which supported glucose-limited growth of the thermoacidophilic organism, Bacillus acidocaldarius, during continuous culture at 55° and pH 3.0, and a hitherto unreported trace element requirement for this organism was established. The effect of temperature (45 - 66°) and pH (2.5 - 5.5) on the growth rate (?max) and growth yield (Yglucose) of B. acidocaldarius during batch culture in this medium was studied using the statistical approach of response surface analysis. This study provided a summary of ?max and Yglucose values throughout the growth region of B. acidocaldarius, from which a more thorough continuous culture study was planned. The growth of B. acidocaldarius was studied during glucose-limited continuous culture over a range of temperature (45 - 64°) and pH (2.8 - 5.5). The molar yield coefficients of such cultures were very much lower than those of mesophilic neutrophiles of similar respiratory chain composition to B. acidocaldarius (Yoz = 28.1 g cells /mol, cf. 61.9 g cells/mol) . Even lower growth yields were observed when the temperature was raised and when the pH was lowered, minimum yields occurring at 64° and pH 2.8 (Yglucose 23.4 g dry wt. cells/mol glucose, Yo2 5.9 g dry wt. cells/mol O2 at D = 0.1 h-1). Several aspects of energy metabolism in these cells were studied. The decreases in growth yield could be correlated with increases in the permeability of the cytoplasmic membrane to protons, i.e. cells needed to catalyse enhanced rates of substrate oxidation in order to avoid a potentially lethal acidification of the cytoplasm. This strategy appears to be successful in that the specific death rates in situ were very low for all cultures except those growing under the most extreme conditions (64° and pH 2.8).