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Relationship of Antibiotic Resistance to Effectiveness in Rhizobium trifolii Populations¹

ABSTRACT

Fifty Rhizobium trifolii isolates were divided into five categories of symbiotic effectiveness and examined for resistance to 15 antibiotics. The cultures were resistant to high concentrations of 11 of the antibiotics but were relatively sensitive to chloramphenicol, streptomycin, tetracycline, and vancomycin. Resistance to any particular group of antibiotics was not associated with any one of the different categories of effectiveness but rather was equally dispersed between all categories. Eighteen isolates which differed in effectiveness were used to develop subcultures resistant to 3x or 4x the average minimum inhibitory concentration (MIC) of the parent strains for each of the antibiotics. Reduction in effectiveness was found in 12.5% of the resistance strains, was not associated with any particular antibiotic, and was randomly distributed throughout the five effectiveness categories. Increased effectiveness occurred with only three subcultures (2.5% of the tests). Cross-resistance patterns were generally restricted to those antibiotics which expressed similar modes of activity and the development of cross-resistance occurred in 5.7% of the possible resistance combinations. Results presented in this study do not agree with other reports in which antibiotics were divided into groups based on their association with effectiveness.

¹ Contribution from the Oregon State Agric. Exp. Stn., Oregon State Univ., Corvallis, OR 97331. Technical Paper no. 4901.

² Associate Professor, Dept. of Agronomy, Mississippi State Univ., Mississippi State, MS 39762; former Assistant Professor, Dept. of Microbiology, Oregon State Univ., Corvallis.

Received for publication December 21, 1978. Accepted for publication March 14, 1979.