Isolation and properties of acetate kinase- and phosphotransacetylase-negative mutants of Thermoanaerobacter thermohydrosulfuricus

Thermoanaerobacter thermohydrosulfuricus (Clostridium thermohydrosulfuricum) ferments carbohydrate substrates to acetate, ethanol and L-lactate. To block acetate production, mutants defective in acetate kinase and/or phosphotransacetylase were isolated after mutagenesis with N-methy-N'-nitro-N-nitroguanidine by selection for fluoroacetate resistance and subsequent counterselection for growth on pyruvate. Upon growth on glucose, all mutants formed only minor amounts of acetate and markedly less ethanol than the wild-type. The mutants produced L-lactate as the main fermentation product, yielding up to 1.8 mol lactate (mol glucose)^-1 as compared to 0.7 mol lactate (mol glucose)^-1 obtained with the parent strain. Expression of the genes encoding acetate kinase and phosphotransacetylase was found to be negatively controlled by acetate.