Effects of organic acids and low pH on Rhizobium meliloti 104A14.

In the symbiotic relationship between Rhizobium meliloti and alfalfa (Medicago sativa), the bacteria are enclosed within the plant cell by a membrane that may function like a plant vacuolar membrane and maintain a pH between 5.5 and 6.0. Free-living Rhizobium meliloti 104A14 is sensitive to pH in this range and its sensitivity was influenced by the presence of acetate and other monocarboxylic acids. R. meliloti can grow at pH 6.0 in 3 mM succinate but does not grow at pH 6.2 if 10 mM acetate is added. The combination of low pH and acetate is bacteriostatic. Measurement of internal pH (pHi) using 14C-labelled benzoate as a permeant acid showed that growth inhibition occurs when pHi falls below 7.15.