Role of population heterogeneity and pH factor in natural phenomenon of α-acetolactate overproduction in Lactococcus lactis ssp. lactis bv. diacetylactis B2103/74 diacetyl producer

Abstract

The influence of population heterogeneity and medium pH factors on the effect of α-acetolactate (AL) overproduction that we found in L. lactis bv. diacetylactis B2103/74 culture was studied. The nature of the enzymatic system responsible for this effect (which is able to considerably block lactate dehydrogenase (LDH) by intercepting up to 80% of the NADH in it) is unknown; we designated this system as the heminindependent electron transferring system (HEMIETS). It was demonstrated that manifestation of the effect was masked by the culture heterogeneity and was especially strong in approximately neutral pH values (6.1–6.5). The presence in the population of at least three types of dissociants differing in the level of HEMIETS activity, and consequently AL synthesis, was established: an active dissociant (AL accumulation up to 30 mM), an inactive dissociant that had lost the HEMIETS (AL no more than 3 mM), and a dissociant with intermediate activity (AL 15–17 mM). It was demonstrated that the HEMIETS activity at a particular pH value had primary importance in the emerging general picture of pyruvate metabolism at different pH values and, finally, in overproduction. Thus, no overproduction effect was observed at a pH of 7.0 due to the low HEMIETS activity, and the pyruvate metabolism passed according to homolactic type. However, the HEMIETS activity was so high at pH values of 5.3–6.5 that this allowed it to take the function of the main regulator of NAD+/NADH oxidation–reduction balance and to give a minor role to LDH. The LDH- and acetolactate synthase pathway productivity depended on the HEMIETS activity at a particular pH, while the productivity of pyruvate dehydrogenase pathway directly depended on the external pH. The latter was relatively high in approximately neutral area (pH 6.0–6.5) and was noticeably decreased in neutral (pH 7.0) and weakly acid (pH ≤ 6.0) zones.