Suppression of lactate production by aerobic fed-batch cultures of Lactococcus lactis

Aerobic fed-batch cultures were studied as a means of suppressing the production of lactate, which inhibits the growth of lactic acid bacteria (LAB). LAB produce lactate via lactate dehydrogenase (LDH), regenerating nicotinamide adenine dinucleotide (NAD⁺) consumed during glycolysis. Therefore, we focused on NADH oxidase (NOX), employing oxygen as an electron acceptor, as an alternative pathway to LDH for NAD⁺ regeneration. To avoid glucose repression of NOX and NAD⁺ consumption by glycolysis exceeding NAD⁺ regeneration by NOX, glucose was fed gradually. When Lactococcus lactis MG 1363 was aerobically fed at a specific growth rate of 0.2 h^-1, the amount of lactate produced per amount of grown cell was reduced to 12% of that in anaerobic batch cultures. Metabolic flux analysis revealed that in addition to NAD⁺ regeneration by NOX, ATP acquisition by production of acetate and NAD⁺ regeneration by production of acetoin and 2,3-butanediol contributed to suppression of lactate production.