An assessment of a method based on intrinsic antibiotic resistance for identifying Rhizobium strains

A method based on intrinsic antibiotic resistance (IAR) for identifying large numbers of Rhizobium strains was assessed and found to be unsatisfactory for R. phaseoli and isolates from Cicer arietinum (Rhizobium spp.). Our data showed that the number of different IAR patterns always exceeded the number of strains tested. With 90 nodule isolates from plants inoculated with a mixture of three strains of R. Phaseoli, the technique gave 18 different resistance patterns. When 24 strains of Rhizobium spp., each replicated three times, were examined 68 different resistance patterns were obtained. Single colony isolates from one strain also gave several different IAR patterns. All strains tested with fluorescent“ antibody were readily identified. Attempts to obtain correct strain identification with IAR by simplifying the scoring systems or allowing up to two differences in the resistance patterns were unsuccessful. We were unable to define the source of this variation although incubation time and inoculum concentration were shown to affect the IAR patterns