Genetic and molecular characteristics of Vir plasmids of bovine septicemic Escherichia coli

Abstract

Three wild strains of bovine septicemic Escherichia coli were selected on the basis of their production of a toxin lethal for mice and chickens and their characteristic surface antigen. The transfer of these virulence (Vir) properties from two of the three to recipient E. coli was detected after mating. One Vir plasmid (pJL1) was derepressed for transfer and associated with mobilization of chromosomal markers. The other, pJL2, was repressed. Both plasmids were tagged with transposon Tn5 (kanamycin resistance), and transfer parameters of the tagged plasmids were studied. The Tn5 insertion in pJL2 usually increased transfer efficiency 100-fold. Plasmid pJL1 was classified as a member of the FIV incompatibility group. A pJL1::Tn5 derivative plasmid was incompatible with ColV1. Plasmid pJL2 behaved as an fi+ plasmid. Both plasmids pJL1 and pJL2 had a molecular weight of 92 x 10(6) and were present at about four copies per chromosome; their deoxyribonucleic acid (DNA) structures were not identical on the basis of restriction enzyme analysis. DNA-DNA hybridization revealed a polynucleotide sequence homology of at least 58% between the two plasmids. No plasmids could be detected in one wild or certain laboratory-derived Vir+ E. coli strains.