Prolonged stability of R plasmids KR61 and KR61-A in Salmonella host

The R plasmids, KR61 and KR61-A, that were originally isolated from a clinical strain of Aerobacter aerogenes in 1971 and determined resistance to kanamycin (Km), neomycin (Nm), streptomycin (Sm), tetracycline (Tc); and ampicillin (Ap) respectively were found stable in Salmonella typhimurium LT2 even after 22 years of cultivation on antibiotic free media. KR61, carrying resistance to KmNmSmTc, not only maintained all its resistances but also maintained its conjugal transferability (RTF) as indicated by its subsequent transfer to Escherichia and Salmonella hosts. KR61-A that carried resistance to Ap and lacked an RTF could be mobilized by KR61 from S. typhimurium LT2, constructed to bear KR61-A and KR61, to E. coli recipients. S. typhimurium LT2 carrying KR61-A + KR61 (ApKmNmSmTc), showed the characteristic conjugal transfer of resistances in following three patterns: (i) Ap, (ii) KmNmSmTc and (iii) ApKmNmSmTc. The findings reported here are based on conjugal isolation of plasmids. Physical isolation of KR61 and KR61-A was never made.