Conversion of Cryptic Plasmids into R Plasmids in Drug Sensitive Escherichia coli Isolated from Laboratory Mice and Rats

Abstract

Of 21 drug sensitive Escherichia coli strains isolated from laboratory mice and rats, 17 harbored 1 to 9 cryptic plasmids. To examine the convertibility of these cryptic plasmids into conjugative R plasmids, we introduced a nonconjugative plasmid, pMK1::Tn2602 (ColE1::Tn5::Tn2602), which encodes resistance to kanamycin (Km) and ampicillin (Ap), into a mouse strain E916 possessing at least 5 plasmids by transformation. A mixed culture of the transformant E 916 (pMK1::Tn2602) as donor and E. coli K-12 strain as the recipient produced Km-resistant and Ap-resistant transconjugants at the frequencies of 10^-9and 10^-6 per donor cell, respectively. Agarose gel electrophoretic analysis revealed that the Km-resistant transconjugant had a single plasmid (pKO916Km) of 29.5 megadaltons (Mdal), which was capable of transforming another strain of E. coli K-12 into a Km-resistant. Similarly, the Ap-resistant transconjugant had a single conjugative R plasmid (pK0916Ap) of 29 Mdal. Restriction enzyme cleavage and electron microscopic analysis demonstrated that these R plasmids were the composite plasmids consisting of a 26 Mdal cryptic plasmid presented in the strain E916 and either of a Km transposon (Tn5) or an Ap transposon (Tn2602). They were conjugally transmissible to different species of enterobacteria such as Shigella and Salmonella. In vivo transfer of pKO916Ap was also demonstrated in gnotobiotic mice.