Change from Homo- to Heterolactic Fermentation by Streptococcus lactis Resulting from Glucose Limitation in Anaerobic Chemostat Cultures

Publisher Website
Google Scholar
Abstract
Lactic streptococci, classically regarded as homolactic fermenters of glucose and lactose, became heterolactic when grown with limiting carbohydrate concentrations in a chemostat. At high dilution rates (D) with excess glucose present, about 95% of the fermented sugar was converted to L-lactate. As D was lowered and glucose became limiting, 5 of the 6 strains tested changed to a heterolactic fermentation such that at D = 0.1 h-1, as little as 1% of the glucose was converted to L-lactate. The products formed after this phenotypic change in fermentation pattern were formate, acetate and ethanol. The level of lactate dehydrogenase, which is dependent on ketohexose diphosphate for activity, decreased as fermentation became heterolactic with S. lactis ML3. Transfer of heterolactic cells from the chemostat to buffer containing glucose resulted in the non-growing cells converting nearly 80% of the glucose to L-lactate, indicating that fine control of enzyme activity is an important factor in the fermentation change. These non-growing cells metabolizing glucose had elevated (about 2-fold) intracellular fructose 1,6-diphosphate concentrations ([FDP]in) compared with those in the glucose-limited heterolactic cells in the chemostat. [FDP]in was monitored during the change in fermentation pattern observed in the chemostat when glucose became limiting. Cells converting 95 and 1% of the glucose to L-lactate contained 25 and 10 mM [FDP]in, respectively. Factors involved in the change to heterolactic fermentation include both [FDP]in and the level of lactate dehydrogenase.