Acidity and calcium interaction affect the growth of Bradyrhizobium sp. and the attachment to peanut roots

The present work was designed to study the effect of acidity and calcium concentration on the Bradyrhizobium sp. strain SEMIA 6144 (peanut symbiont). The bacteria showed a growth and viability diminution at low pH (5.0) and at a calcium concentration of 0.05 mM. Increasing concentrations of calcium significantly improved the rhizobial growth under acid stress conditions. Molecules related to the plant-bacteria recognition, such as exopolysaccharides (EPS) and lipopolysaccharides (LPS) showed changes at different pH values and calcium concentrations. Variations in the cellular permeability were also observed under stress conditions. Moreover, the attachment of the bacteria to excised peanut roots was examined. Optimal bacteria root attachment to excised peanut roots was dependent on pH and calcium limitation in the medium.