The effect of temperature and pH on the growth efficiency of the thermoacidophilic bacterium Bacillus acidocaldarius in continuous culture

Abstract

The molar yield coefficients (Y _glucose, Y _{O 2}) of glucose-limited continuous cultures of the thermoacidophile Bacillus acidocaldarius have been measured as a function of dilution rate as well as over a range of temperature and pH (51°C to 64°C, pH 2.8–5.5) at a fixed dilution rate of approximately 0.1 h^-1. The highest growth yields were observed at 51°C and pH>4.3 (Y _glucose 54.8 g cells · mol glucose^-1, Y _{O 2} 15.0 g cells · mol O ^-1₂ ), but were very much lower than those of mesophilic neutrophiles of similar respiratory chain composition to B. acidocaldarius. Even lower growth yields were observed when the temperature was raised or when the pH was lowered, lowest yields occurring at 64°C and pH 2.8 (Y _glucose 23.4 g cells · mol glucose^-1, Y _{O 2} 5.9 g cells · mol O ^-1₂ ).

These 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°C, pH 2.8).