ASSOCIATION BETWEEN ANTIBIOTIC RESISTANCE AND INEFFECTIVENESS IN MUTANT STRAINS OF RHIZOBIUM SPP.

Abstract

Loss of effectiveness was found to be closely associated with mutation to viomycin resistance and neomycin resistance in strains representing three cross-inoculation groups of Rhizobium—R. leguminosarum, R. trifolii, and R. meliloti. Twenty-nine of 33 mutant clones, selected from 11 normally effective parent strains for resistance to viomycin, were uniformly ineffective. Both viomycin resistance and ineffectiveness remained unchanged in clones reisolated from nodules. Fifteen of 16 clones selected for neomycin resistance from eight normally effective strains were also ineffective, fn contrast, loss of effectiveness occurred only infrequently in clones resistant to kanamycin or polymyxin, and was not observed at all in streptomycin-resistant mutants. Cytologically, ineffective nodules were characterized by rapid dissolution of bacteroid-containing tissue. Ability to form nodules (infectiveness) on the homologous host was retained in all antibiotic-resistant mutants. Full cross resistance was noted for strains selected for resistance to viomycin and neomycin. Kanamycin resistance conferred resistance to neomycin but not to viomycin. Implications of the marker-associated changes are considered in relation to some problems in Rhizobium genetics and physiology.