Streptococcus faecalis mutants defective in regulation of cytoplasmic pH

We have isolated two acid-sensitive mutants of Streptococcus faecalis (ATCC 9790), designated AS13 and AS25, which grew at pH 7.5 but not at pH below 6.0. The ionophore gramicidin D, which collapsed the pH gradient between the cytoplasm and the medium, had little effect on the growth of these mutants, indicating that growing cells maintain only a small pH gradient. In the presence of gramicidin D the growth rates of the parent and mutant strains were identical over a range of pH values. When glucose was added to a cell suspension at pH 6.4, the parent strain generated a pH gradient of 1.0 unit, interior alkaline; AS13 generated a pH gradient of only 0.5 units, and AS25 generated no measurable pH gradient. The proton permeability of the mutant strains was the same as that of the parent strain. These results suggest that a cytoplasmic pH of around 7.5 is required for the growth of the cells and that the mutant strains are unable to establish a neutral cytoplasmic pH in acidic medium because of damage to the regulatory system of the cytoplasmic pH. Mutant strains also have a reduced capacity to extrude protons and take up potassium. Therefore, it is likely that these cation transport systems are involved in the regulation of cytoplasmic pH.