The Journal of Medical Microbiology, Vol 18, Issue 1 95-105, Copyright © 1984 by Society for General Microbiology
Distribution and transferability of plasmids in trimethoprim-resistant urinary strains of Escherichia coli: a comparative study of hospital isolates
C. A. Kraft, D. J. Platt and M. C. Timbury
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.
