Measurement of yeast intracellular pH by image processing and the change it undergoes during growth phase.

The intracellular pH of the yeast Saccharomyces cerevisiae was determined by a fluorescence microscopic image processing technique. Image processing was carried out using a modification of the ratio imaging method for measurement of yeast intracellular pH. Care was necessary when taking fluorescence images in order to obtain accurate measurement of yeast intracellular pH. Until now it has been difficult to measure the intracellular pH of cells in actual cultivation conditions. This method enabled us not only to measure the intracellular pH of dilute cell suspensions, but also to obtain two-dimensional information. In the case of resting cells, the intracellular pH was dependent upon the extracellular pH, and this value was constant when the extracellular pH was constant. On the other hand, in the case of actively growing cells, intracellular pH was found to change, even if the extracellular pH was constant: the values observed were intracellular pH 5.7 during lag phase, intracellular pH 6.8 during exponential phase and intracellular pH 5.5 during stationary phase. These results for intracellular pH indicate that the yeast proton pump was activated during growth from the point of view of pH in vivo.