ACID PH TOLERANCE IN STRAINS OF RHIZOBIUM AND BRADYRHIZOBIUM, AND INITIAL STUDIES ON THE BASIS FOR ACID TOLERANCE OF RHIZOBIUM-TROPICI UMR1899

Acid pH limits the persistence of Rhizobium strains in soil, and the n odulation and nitrogen fixation of legumes. To identify acid-tolerant strains, we tested the ability of 45 Rhizobium, Azorhizobium, and Brad yrhizobium 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 Bladyrh izobium strains grew at pH 4.25, and 15 strains grew at pH 4.50. Toler ance to acid pH in R. tropici UMR1899 was not an adaptive response, no r was it plasmid mediated, correlated with the production of extracell ular polysaccharide, or related to synthesis of polyamines in the cell . When UMR1899 was grown in buffered medium at acid pH, it maintained a Delta pH (measured using P-31-NMR) of up to 1.7 pH units. However, w hen this strain was subjected to acid shock, it showed only limited ab ility to regulate cytoplasmic pH in the short term. Cells of UMR1899 a ccumulated glutamate under pH stress, and were markedly hydrophobic an d resistant to the effects of crystal violet, the latter traits raisin g the possibility that outer membrane composition and structure could also be a factor in pH tolerance.