Transfer of trimethoprim resistance from fecal Escherichia coli isolated during a prophylaxis study in Mexico.

Forty-one of 100 trimethoprim-resistant fecal isolates of Escherichia coli isolated during a diarrhea prophylaxis study transferred trimethoprim resistance to an E. coli K12 derivative at frequencies ranging from 1 x 10(-7) to 2 x 10(-2). Analysis of these transconjugants, after attempting to select those that had received only one plasmid, revealed that 34 (83%) were resistant to trimethoprim plus only one or two of seven other antimicrobial agents tested, most often ampicillin and/or streptomycin. In contrast, 91% of the donor strains were resistant to four or more antimicrobial agents. Plasmid analysis demonstrated that a 35-megadalton plasmid was present in 35 transconjugants and was usually associated with resistance to trimethoprim, streptomycin, and ampicillin; 19 transconjugants had only a 35-megadalton plasmid and were resistant to only these three antimicrobial agents, raising the possibility that a common plasmid was present in many strains. Although resistance to multiple antimicrobial agents could be transferred to the K12 recipient, resistance to more than three agents was seldom mediated by a single plasmid. The high degree of transferability of trimethoprim resistance from these fecal isolates of E. coli emphasizes the potential for the dissemination of genes carrying trimethoprim resistance.