Dynamic changes of intracellular pH in individual lactic acid bacterium cells in response to a rapid drop in extracellular pH

We describe the dynamics of changes in the intracellular pH (pH(i)) values of a number of lactic acid bacteria in response to a rapid drop in the extr acellular pH (pH(ex)). Strains of Lactobacillus delbrueckii subsp. bulgaric us, Streptococcus thermophilus, and Lactococcus lactis were investigated. L isteria innocua, a gram-positive, non-lactic acid bacterium, was included f or comparison. The method which we used was based on fluorescence ratio ima ging of single cells, and it was therefore possible to describe variations in pH(i) within a population, The bacteria were immobilized on a membrane f ilter, placed in a closed perfusion chamber, and analyzed during a rapid de crease in the pH(ex) from 7.0 to 5.0, Under these conditions, the pH(i) of L. innocua remained neutral (between 7 and 8), In contrast, the pH(i) value s of all of the strains of lactic acid bacteria investigated decreased to a pproximately 5.5 as the pH(ex) was decreased. No pronounced differences wer e observed between cells of the same strain harvested from the exponential and stationary phases. Small differences between species were observed with regard to the initial pH(i) at pH(ex) 7.0, while different kinetics of pH( i) regulation were observed in different species and also in different stra ins of S. thermophilus.