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

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. However, as D was lowered and glucose became limiting, five of the six 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 upon ketohexose diphosphate for activity, decreased as fermentation became heterolactic with Streptococcus lactis ML3. Transfer of heterolactic cells from the chemostat to buffer containing glucose resulted in the nongrowing 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 nongrowing cells metabolizing glucose had elevated (ca. twofold) intracellular fructose 1,6-diphosphate concentrations ([FDP]m.) compared with those in the glucose-limited heterolactic cells in the chemostat. [FDP]ui 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. It is suggested that factors involved in the change to heterolactic fermentation include both [FDP]in and the level of lactate dehydrogenase. Group N streptococci (Streptococcus cremoris, S. lactis, and S. diacetylactis) play a vital role in many commercial milk fermentations, in which their primary function is to convert lactose to lactic acid (18). Lactic streptococci are useful because they possess limited metabolic diversity and usually convert about 95% of the fermented sugar to L-lactate (23, 26). This homolactic fermentation of either lactose or glucose occurs in batch culture when organisms are grown anaerobically near pH 7 at 30°C. In contrast, heterolactic fermentation was observed during growth on galactose (26) and during growth on lactose of variants defective in either lactate dehydrogenase (LDH; 21) or the lactose phosphotransferase system and/or phospho-fB-D-galactosidase (7, 26), suggesting that these organisms have pathways which are not normally expressed. The alternative products reported include acetate, acetoin, C02, ethanol, formate, and glycerol. Unsuccessful attempts

Citrate metabolism in lactic acid bacteria

“…(1) lactate dehydrogenase: pyruvate + NADH -~ r,-lactate + NAD ( Fig. 1A) (2) pyruvate formate lyase: pyruvate --* acetyl-P + formate ( Shifts in product formation from lactose, glucose and pyruvate upon aeration, changing of the pH or cultivation under carbohydrate limitation are generally observed in different homofermentative lactic acid bacteria [50][51][52][53]. The complex mechanisms that are involved in regulation of pyruvate metabolism L. lactis will be discussed below.…”

Organization and regulation of genes for amino acid biosynthesis in lactic acid bacteria

“…The study of the nutritional requirements of lactic acid bacteria has been made possible following the early development of chemically defined media allowing growth of Lactococcus lactis [7][8][9][10][11] or various species of lactobacilli including some of those used in food fermentations [12][13][14][15]. All these media contain, in addition to mineral salts and glucose, amino acids, vitamins and nu-cleic acid bases.…”

Responses of lactic acid bacteria to oxygen

“…For example, several lactic streptococci accumulate substantial amounts of formate, ethanol and acetate and correspondingly less lactate when grown in glucose-or lactose-limited unaerated chemostat cultures, whereas with excess sugar the same cultures are homolactic. The production of products other than lactate in the sugar-limited cultures is attributed to relatively low intracellular levels of fructose-l,6-bisphosphate (FBP), an essential activator of lactate dehydrogenase in these strains [60]. The presence of 0 2 often has significant effects on the metabolism of pyruvate in these and other LAB.…”

“…The operation of this pathway varies among LAB. Thomas et al [60] found that most Group N streptococci produced formate, acetate and ethanol from glucose or lactose in unaerated chemostat cultures only when the sugar was limit-ing and FBP levels were low. However, some oral streptococci accumulate these end-products even when glucose is in excess in strictly anaerobic conditions.…”