Evidence for a slowing in trimethoprim resistance during 1981--a comparison with earlier years

Two thousand seven hundred bacterial strains, isolated during 1981 from urine specimens both from patients in hospital and general practice, were examined for resistance to trimethoprim. The incidence of such resistance was 13% in the hospital isolates and 5.8% in the general practice strains. High level resistance (MIC greater than 1 mg/ml) was present in 75-90% of the resistant strains. With the exception of Staphylococcus epidermidis and Proteus mirabilis, almost all the trimethoprim-resistant strains were also resistant to sulphonamide. In 410 strains from sputum specimens the incidence of resistance was 5.4%. Disc testing for the determination of sensitivity of urine isolates is best carried out using a 5 micrograms disc, while for sputum isolates our present experience indicates that a 1.25 micrograms disc is most appropriate. Our results indicate that the rate of increase in the acquisition of resistance to trimethoprim appears to have slowed down compared with the period 1975-1977, and that almost all this resistance is now high level. The introduction of trimethoprim alone for therapeutic use appears to have little if any effect on the incidence of trimethoprim resistance.