Acid pH tolerance in strains of Rhizobium and Bradyrhizobium, and initial studies on the basis for acid tolerance of Rhizobium trpici UMR1899

Acid pH limits the persistence of Rhizobium strains in soil, and the nodulation and nitrogen fixation of legumes. To identify acid-tolerant strains, we tested the ability of 45 Rhizobium, Azorhizobium, and Bradyrhizobium strains to produce isolated colonies on agar medium of pH 4.00 to pH 7.00. Only Rhizobium tropici UMR1899 (=CIAT899) grew at pH 4.00 in unbuffered medium, though 6 strains of R. tropici and 3 Bradyrhizobium strains grew at pH 4.25, and 15 strains grew at pH 4.50. Tolerance to acid pH in R. tropici UMR1899 was not an adaptive response, nor was it plasmid mediated, correlated wiih the production of extracellular polysaccharide, or related to synthesis of polyamines in the cell. When UMR1899 was grown in buffered medium at acid pH, it maintained a ΔpH (measured using ³¹P-NMR) of up to 1.7 pH units. However, when this strain was subjected to acid shock, it showed only limited ability to regulate cytoplasmic pH in the short term. Cells of UMR1899 accumulated glutamate under pH stress, and were markedly hydrophobic and resistant to the effects of crystal violet, the latter traits raising the possibility that outer membrane composition and structure could also be a factor in pH tolerance.