Occurrence of the vir plasmid among animal and human strains of invasive Escherichia coli.

Forty-six isolates of Escherichia coli recovered from systemic diseases in animals and persons were studied to determine which isolates possessed properties attributed to the Vir plasmid. These properties were production (i) of a chick letha toxin and (ii) of a specific surface antigen. Neither toxic activity nor surface antigen production could be detected in nine E coli strains from septicemia in chickens and 11 from human cerebrospinal fluid. Two of 8 strains from human septicemic disease and 3 of 18 isolates from calf septicemia produced the toxin, but lacked the characteristic surface antigen. Two of the 18 bovine strains had both characteristics. Several methods of screening for Vir+ transconjugants were investigated in an attempt to facilitate their recognition after mating a Vir+ donor with a suitable recipient. These methods, which included precipitation in agar plates containing antiserum, immunodiffusion tests, and fluorescent antibody tests, were unsuccessful. It was observed, however, that the pattern of growth in tryptic soy broth of the Vir+ transconjugants was usually different from that of the Vir- clones. Vir+ transconjugants grew as a heavy sediment with a clear supernatant, whereas their Vir- counterparts showed a uniform turbidity. When the Vir plasmid was transferred to an E coli strain which lacked appendages, fimbriae were observed to be associated with the presence of the plasmid. Two types of fimbriae were evident. One type appeared to be sex fimbriae and the other was probably the surface antigen associated with the Vir phenotype.