Abstract
During studies on the transferable antibiotic resistance of clinical gram-negative bacteria, 28 R plasmids were isolated from indigenous strains. The R plasmids conferred resistance to one or several antibiotics on their host strains and most of them were conjugally transferable to other pathogenic bacteria, posing a threat to chemotherapy. Some of the R plasmids were conjugally nontransferable. It was of interest to see whether they could become transferable to other bacteria in the presence of a co-resident conjugative plasmid. For this purpose, genetically marked strains of Escherichia coli, carrying the individual, non-transferable R plasmids, were conjugally infected by the conjugative plasmids: F′ pro+ lac+ , KR61-KNST, pSK1a, pAK3, pAK6, pAK13 and pAK17. Mobilization of the non-transferable R plasmids was observed in different patterns by the co-existing conjugative plasmids in conjugal crosses.
