The physiology of Lactococcus lactis subsp. lactis biovar. diacetylactis immobilized in hollow-fibre bioreactors: glucose, lactose and citrate metabolism at high cell densities

Abstract

Lactococcus lactis subsp, lactis biovar, diacetylactis was selected to study the physiological influences of immobilization and growth to high cell densities. Cells were cultivated on glucose or lactose medium in the presence and absence of citrate. With excess glucose the cells produced mainly lactate as the fermentation product (homofermentative) providing that not all of the substrate was consumed. The population so cultivated was exposed to extreme gradients of pH and lactate concentrations. When the glucose concentration was reduced the population showed a mixed product profile with half of the glucose being fermented to lactate, the remainder to formate, acetate, ethanol and 2,3-butanediol. Inclusion of citrate in the medium shifted the population to homofermentation, with respect to the amount of glucose or lactose consumed. The citrate was metabolized via the pyruvate-formate lyase and o~-acetolactate synthase routes. The pH of the medium was shown to strongly influence the product profile from citrate, presumably by affecting the activity of the key enzymes of pyruvate metabolism. The lactococci immobilized at high cell densities show product profiles typical of carbohydrate limitation at low dilution rates.

Citrate metabolism in lactic acid bacteria

“…Below pH 5.0 citrate utilization is low in these microorganisms due to low activity of the citrate permease and low metabolic activity in general. Resting cells of L. lactis were shown to utilize citrate at pH levels as low as 4.5 [16] and when immobilized in a hollow-fibre reactor some activity was observed at pH values down to pH 4.0 [17]. For a few lactic acid bacteria citrate utilization has been reported at even lower pH.…”

“…Also, within the homofermentative lactic acid bacteria complete different strategies are observed for citrate conversion. In Lactobacillus pentosus and Lactobacillus plantarum citrate conversion to succinate (and acetate) was reported [21,22], while in Lactococcus lactis acetoin, diacetyl, formate and acetate are the main products from citrate metabolism [14,17,40]. Even within one species, large variations between strains are observed.…”