Selection for acid tolerance inRhizobium meliloti

In the selection of acid-tolerantRhizobium meliloti, procedures for the collection and isolation of rhizobia, and the assessment of acid tolerance, have not been critically evaluated. Such procedures form the basis of this study. Root nodules were collected fromMedicago spp. found growing on acid soil in Sardinia. Their encumbent bacteria were isolated directly onto media adjusted over a range of pH values, and then assessed for acid tolerance in both the laboratory and field. Strains ofRhizobium meliloti isolated onto low pH media were, in general, more acid-tolerant than sister isolates from high pH media, when tested in both the laboratory and field. Dilution (10 or 100 fold) of the inocula used in the laboratory assessment did not greatly influence the rating derived, although there was some effect of bacterial colony type on growth rating. The link between polysaccharide production and acid tolerance was not strong. There was a poor correlation between the growth ratings derived from the laboratory screening and acid tolerance as expressed in the field.