Acid Sensitivity of a Mutant of Lactococcus lactis subsp. lactis C2 with Reduced Membrane-bound ATPase Activity

Abstract

Mutants with reduced membrane-bound ATPase activities were isolated from Lactococcus lactis subsp. lactis C2 as spontaneous neomycin-resistant mutants. Characteristics of the representative mutant, No. 1016-51, were compared with the parental strain in cultures using a jar fermentor with the pH controlled at various values. At pH 6. 5, the fermentation patterns, i. e., glucose consumption, growth, and lactic acid production, of both strains appeared identical. At pH 4. 5, however, the levels of growth, lactic acid production, and the amounts of lactic acid produced per cell after the culture for 24h decreased to 60, 36, and 60% of the parental strain, respectively. During the cultures at pH 6. 5, no differences were found in viabilities between both strains even after 80h. On the other hand, at pH 4. 0, the viable count of the strain No. 1016-51 in a 72-h culture decreased to less than 1% of that of the zero time, while the parental strain maintained its original viability. Therefore, it was concluded that the membrane-bound ATPase is essential for this organism to survive at low pH, probably through its function of proton pumping for maintaining cytoplasmlc pH levels.