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[Instability of natural, multiple drug resistance in actinomycetes].

Resistance to tetracycline, chloramphenicol, penicillin, polymyxin, erythromycin, lincomycin and ristomycin in a number of strains of Str. coelicolor A3 (2) was studied. It was found that their resistance to the above antibiotics was not associated with plasmids SCP1 and SCP2. The frequency of spontaneous variants sensitive to one and several antibiotics amounted in Str. coelicolor to 0.1--0.3 per cent. Ethidium bromide had no significant effect on the frequency of the sensitive variants. However, after growth of the culture at 37 degrees C the number of the variants sensitive to tetracycline and polymyxin significantly increased. The sensitive variants of Str. coelicolor A3 (2) formed resistant revertants at various frequencies (10(-2)--10(-6)). The frequency of variants of Str. rimosus 183 simultaneously sensitive to kanamycin, monomycin, neomycin and chloramphenicol was high. They also reverted to the resistant ones with a frequency of about 10(-3). The results of the study are indicative of the genetic instability of the antibiotic natural resistance in some actinomycetous strains. Such instability is probably not connected with the plasmid elimination.