Genetic and physiological analysis of the lethal effect of L-(+)- lactate dehydrogenase deficiency in Streptococcus mutans: Complementation by alcohol dehydrogenase from Zymomonas mobilis

CH4ts is a previously isolated recombinant mutant of Streptococcus mutans NG8 which produces a thermolabile L-(+)-lactate dehydrogenase (LDH) activity. It does not grow at 42°C under a variety of cultivation conditions. In this study, we show that a batch culture of CH4ts shifted from 30 to 42°C underwent rapid cessation of growth and accelerated cell death. The mutant grew at 42°C in continuous culture under glucose-limiting conditions. Under these conditions, lactate production was replaced by production of ethanol and, to a smaller extent, acetoin. The cloned Zymomonas mobilis gene for alcohol dehydrogenase II, placed under the control of the S. mutans spaP regulatory signals, complemented LDH deficiency. The alcohol dehydrogenase-complemented mutant grew as well or better than NG8 on a variety of carbon sources at 42°C and produced significant amounts of ethanol in place of lactic acid. These results are in accord with other approaches indicating that S. mutans has other enzymatic activities, including pyruvate formate-lyase and pyruvate dehydrogenase, for pyruvate metabolism. However, at high glucose concentrations, the levels of activity of these enzymes are apparently insufficient to compensate for the absence of LDH.