The ability of lactic acid bacteria to regulate their cytoplasmic or intracellular pH is one of the most important physiological requirements of the cells. Cells unable to maintain a near neutral intracellular pH during growth or storage at low extracellular pH may lose viability and cellular activity. Despite the importance of pH homeostasis in the lactic acid bacteria, however, an understanding of cytoplasmic pH regulation has only recently begun to emerge. This review describes the specific effects of low pH on lactic acid bacteria, reports recent research on the physiological role of intracellular pH as a regulator of various metabolic activities in lactic acid bacteria, and presents the means by which lactic acid bacteria defend against low intracellular pH. Particular attention is devoted to the proton-translocating ATPase, an enzyme that is largely responsible for pH homeostasis in fermentative lactic acid bacteria. (Key words: pH, pH homeostasis, lactic acid bacteria) Abbreviation key: pHout = extracellular pH, pHi, = intracellular pH, ApH = pH gradient, H+-ATPase = proton-translocating ATPase.
The objectives of this research were to determine the effect of lactic acid and low pH on the intracellular pH in three species of lactic acid bacteria. A pH gradient (intracellular pH minus the extracellular pH) of .9 to 1.4 pH units was achieved by several strains of lactic acid bacteria, including Streptococcus thermophilus 19258 and 573, Lactococcus lactis ssp. lactis C2, and Lactococcus lactis ssp. cremoris HP during log phase of growth in various media. A noticeable decline of the pH gradient occurred at an internal pH of 5.5 to 6.0. In late stationary phase, the pH gradient was generally reduced to .5 pH units or less. In contrast, the aciduric Lactobacillus casei 685 maintained a large pH gradient (> 1.0 pH units) even when the medium pH was reduced to less than 4.0. Rapid growth of lactococci and streptococci in media containing excess lactose did not occur when the intracellular pH was reduced below a critical pH of 5.0 or at a neutral pH when proton-uncoupling agents were present. (Key words: lactic acid bacteria, intracellular pH, lactic acid) Abbreviation key: CCCP = carbonyl cyanide m-chlorophenylhydrazone, ApH = pH gradient (intracellular pH minus extracellular pH), PEG = [3HJpolyethylene glycol, pHh = intracellular pH, pHout = extracellular pH, SMM = simulated milk medium.
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