Distribution and transferability of plasmids in trimethoprim-resistant urinary strains of Escherichia coli: a comparative study of hospital isolates.

Trimethoprim-resistant urinary isolates of Escherichia coli, collected in 1982, were studied and the results compared with those obtained for isolates collected during the period 1979-1980. Ninety-eight (81%; a 10% increase) were resistant to trimethoprim 1024 micrograms/ml and 93 (95%) were also resistant to sulphamethoxazole 1024 micrograms/ml. The frequency distributions of plasmids were similar in both collections although there was a significant increase in the number of small plasmids (mol. wt less than or equal to 20 X 10(6] in the 1982 collection. Transfer of resistance was associated with isolates that carried larger numbers of plasmids. A significantly smaller proportion of isolates in this, than in the earlier series, transferred trimethoprim resistance to E. coli K12 suggesting continued transposition of trimethoprim resistance on to the bacterial chromosome. Fifteen different trimethoprim resistance plasmids were identified, of which five, including that found most frequently, were common to both collections. Plasmids which transferred trimethoprim resistance without sulphonamide resistance were more common in the 1982 series. Plasmids which transferred linked trimethoprim and streptomycin resistances, in particular those not carrying other resistance markers, were less common in the 1982 series of isolates. Although the majority of E. coli isolates highly resistant to trimethoprim remained resistant to sulphonamide, our results suggested changes in the genetic location and linkage of the resistance markers.