Dilworth Michael J, Rynne Fiona G, Castelli Joanne M, Vivas-Marfisi Ana I, Glenn Andrew R
Microbiology (Reading). 1999 Jul;145 ( Pt 7):1585-1593. doi: 10.1099/13500872-145-7-1585.
Cells of Sinorhizobium meliloti WSM419 showed an adaptive acid-tolerance response 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 WSM419 cells under conditions of acid stress (pH 4.0). The effect of calcium on survival at pH 4.0 however, appears unconnected to exopolysaccharide (EPS), 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 survival at pH 4.0, and the effect varied with pH. In cells grown at pH 5.8, higher calcium concentrations also markedly increased the rate of synthesis of EPS; this was not seen in cells grown at pH 7.0. 1H NMR spectra for isolated 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 concentration did not affect the pattern of substitution at either pH. For EPS to be involved in the effect of calcium concentration in the growth medium on survival would imply a deleterious effect of the EPS produced at low pH.
与在pH 7.0条件下生长相比，苜蓿中华根瘤菌WSM419细胞在pH 5.8条件下生长时表现出适应性耐酸反应。在酸胁迫（pH 4.0）条件下，增加暴露培养基中钙的浓度可显著降低WSM419细胞的死亡率。然而，钙对pH 4.0条件下细胞存活的影响似乎与胞外多糖（EPS）无关，因为exoY（Rm0540）发生突变的菌株对暴露培养基中钙的反应与其野生型亲本（Rm2011）相同。生长培养基中钙的浓度也会影响随后在pH 4.0条件下的细胞存活，且这种影响随pH值而变化。在pH 5.8条件下生长的细胞中，较高的钙浓度也显著提高了EPS的合成速率；而在pH 7.0条件下生长的细胞中则未观察到这种现象。对在pH 5.8和pH 7.0条件下生长的WSM419培养物中分离出的EPS进行的1H NMR光谱分析表明，低pH值显著降低了乙酰基和丙酮酰基的取代程度，但未降低琥珀酰基的取代程度；钙浓度在两种pH值条件下均未影响取代模式。若EPS参与生长培养基中钙浓度对细胞存活的影响，这将意味着在低pH值条件下产生的EPS具有有害作用。
