Acidity and calcium interaction affecting cell envelope stability in Rhizobium

Calcium improves the ability of many rhizobia to survive and persist in acid soils, but the mechanism responsible for this phenomenon has not been studied in detail. Here, we present data examining the combined effects of pH and calcium on the cell envelope of Rhizobium strains that differ in pH tolerance. The effect of pH and calcium on solute uptake was demonstrated by a change in the resistance to selected antimicrobial agents. When grown at pH 5.0, all strains exhibited fatty acid methyl ester profiles that were significantly different from those obtained using cells grown at pH 7.0. These differences included changes in the C16:C18 ratio and the percentage of 19:0 cyclopropane in the membrane. Both pH and calcium level had marked effects on Rhizobium etli UMR1632 lipopolysaccharide-banding patterns, but there was little evidence of a change in lipopolysaccharides with pH and calcium in Rhizobium tropici UMR1899. Both pH and calcium influenced expression of outer membrane proteins in all strains.