R-plasmid transfer in vivo in the absence of antibiotic selection pressure.

The persistence of an O18 Escherichia coli strain resistant to tetracycline, streptomycin, and sulfonamide has been followed in the fecal flora of a single individual over a period of 9 months. The strain in question carrying an R plasmid was detectable from the beginning of the survey, but it was only after a 10-day period of tetracycline therapy that it reached an all but permanent dominance in the fecal flora. No transfer of the R plasmid carried by the strain to any other coliform could be detected for 202 days after the end of tetracycline treatment. At this point, however, an O88 E. coli carrying the same plasmid as the O18 strain appeared briefly as a predominant component of the flora. The two plasmids isolated from the O18 and the O88 E. coli strains have been characterized in molecular terms and found to be similar. This suggests that R-plasmid transfer between two E. coli strains occurred in an individual who was living a normal daily life and who was not receiving antibiotics.