Relationship between acid tolerance, cytoplasmic pH, and ATP and H+-ATPaselevels in chemostat cultures of Lactococcus lactis

The acid tolerance response (ATR) of chemostat cultures of Lactococcus lact is subsp, cremoris NCDO 712 was dependent on the dilution rate and on the e xtracellular pH (pH(o)), A decrease in either the dilution rate or the pH(o ) led to a decrease in the cytoplasmic pH (pH(i)) of the cells, and similar levels of acid tolerance were observed at any specific pH(i) irrespective of whether the pH(i) resulted from manipulation of the growth rate, manipul ation of the pH(o), or both. Acid tolerance was also induced by sudden addi tions of acid to chemostat cultures growing at a pH(o) of 7.0, and this ind uction was completely inhibited by chloramphenicol, The end products of glu cose fermentation depended on the growth rate and the environmental pH(o) o f the cultures, but neither the spectrum of end products nor the total rate of acid production correlated with a specific pH(i). The rate of ATP forma tion was not correlated with pH(i), but a good correlation between the cell ular level of H+-ATPase and pH(i) was observed. Moreover, an inverse correl ation between the cytoplasmic levels of ATP and pH(i) was established. Each pH(i) below 6.6 was characterized by unique levels of ATR, H+-ATPase, and ATP, High levels of Bt ATPase also coincided with high levels of acid toler ance of cells in batch cultures induced with sublethal levels of acid. We c oncluded that H+-ATPase is one of the ATR proteins induced by acid pH(i) th rough growth at an acid pH(o) or a slow growth rate.