Monitoring the intracellular pH of Zygosaccharomyces bailii by green fluorescent protein

It is generally known that intracellular pH (pHi) plays a vital role in cell physiology and that pHi homeostasis is essential for normal cellular functions. Therefore, it is desirable to know the pHi during cell life cycle or under various growth conditions. Different methods to measure pHi have been developed and among these methods, the use of pH-sensitive green fluorescent protein (GFP) as a pHi indicator is a promising technique. By using this approach, not only can more accurate pHi results be obtained but also long-term experiments on pHi can be performed. In this study, the wild type Zygosaccharomyces bailii, a notorious food spoilage yeast, was transformed with a plasmid encoding a pH-sensitive GFP (i.e. pHluorin), enabling the pHi of the yeast to be determined based on cellular fluorescent signals. After the transformation, growth and pHi of the yeast were investigated in four different acidic conditions at 22 °C during 26 days. From the experimental results, the transformation effectiveness was verified and a good correlation between yeast growth and pHi was noticed. Particularly, it was observed that the yeast has an ability to tolerate a significant pHi drop during exponential phase and a subsequent pHi recovery in stationary phase, which may underlie the exceptional acid resistance of the yeast. This was the first time that a GFP-based approach for pHi measurement was applied in Z. bailii and that the pHi of the yeast was monitored during such a long period (26 days). It can be expected that greater understanding of the physiological properties and mechanisms behind the special acid resistance of the yeast will be obtained from further studies on this new yeast strain.