Survival and exopolysaccharide production in Sinorhizobium meliloti WSM419are affected by calcium and low pH

Cells of Sinorhizobium meliloti WSM419 showed an adaptive acid-tolerance re sponse when grown at pH 5.8 instead of pH 7.0. Increasing concentrations of calcium in the exposure medium significantly decreased the death rate of W SM419 cells under conditions of acid stress (pH 4.0). The effect of calcium on survival at pH 4.0 however, appears unconnected to exopolysaccharide (E PS), since a strain with a mutation in exoY(Rm0540) responded to calcium in the exposure medium in the same way as its wild-type parent (Rm2011). The concentration of calcium in the growth medium also affected subsequent surv ival at pH 4.0, and the effect varied with pH. In cells grown at pH 5.8, hi gher calcium concentrations also markedly increased the rate of synthesis o f EPS; this was not seen in cells grown at pH 7.0. H-1 NMR spectra for isol ated EPS from WSM419 cultures grown at pH 5.8 and pH 7.0 showed that low ph markedly lowered the degree of substitution with acetyl and pyruvyl groups . but not the degree of substitution with succinyl groups; calcium concentr ation did not affect the pattern of substitution at either ph. For EPS to b e involved in the effect of calcium concentration in the growth medium on s urvival would imply a deleterious effect of the EPS produced at low ph.