Changes in acid tolerance of Lactococcus lactis during growth at constant pH

Cells of Lactococcus lactis MG1363 growing in batch culture in TYG (trypton e, yeast extract, glucose) medium at constant pH 7.0 became gradually more acid sensitive shortly after inoculation until a point of maximum sensitivi ty was reached in early log-phase. The acid tolerance then gradually increa sed in the mid- and late-log phase until maximum tolerance was reached at t he onset of stationary phase. This pattern has been termed the growth-phase acid tolerance. The variation in acid tolerance seen in pH 7.0 grown cells of L. lactis MG1363 did not result from changes in internal pH or membrane H+ ATPase activity levels. Neither the amount of glucose present during mi d-log phase nor the amount of lactate produced by the cells correlated with the pattern of the log-phase acid tolerance. Cells grown in partially spen t TYG medium showed a reduced growth rate and increased acid tolerance comp ared to cells grown in fresh TYG medium. Supplementing the spent medium wit h tryptone or yeast extract or both restored the growth rate and cells beca me more sensitive to acid. Fractionation of tryptone yielded a fraction whi ch stimulated the growth of MG1363 in partially spent medium and delayed th e acquisition of acid tolerance. The active compound(s) has a putative mole cular weight of about 1 kDa and was partially degraded by papain and trypsi n. (C) 2000 Elsevier Science B.V. All rights reserved.