Isolation of recombinants doubly and triply drug-resistant to streptomycin, tetracycline and chloramphenicol by PEG-induced cell fusion of singly resistant staphylococcus aureus L-forms.

Various combinations of four substrains of Staphylococcus aureus L-form (strain STA-EMT-1), each of which was resistant to one of the following four drugs, streptomycin (SM), tetracycline (TC), chloramphenicol (CP) and erythromycin (EM), were submitted to polyethylene glycol (PEG)-induced cell fusion. PEG-induced cell fusion followed by enrichment culture in the liquid basal medium supplemented with penicillin G resulted in development of recombinants that were doubly drug-resistant to SM and TC, SM and CP, and TC and CP, but no recombinant doubly resistant to EM and TC, was obtained by treatment of a EM-resistant and TC-resistant substrains with PEG. No recombinants resistant to SM, CP and TC could be obtained by treatment of substrains resistant to SM, CP and TC, respectively, with PEG. But recombinants triply resistant to these three drugs were produced by two-step cell fusion; that is by fusion of a recombinant doubly resistant to two of the three drugs with a substrain resistant to the third drug.