TRIMETHOPRIM-R PLASMIDS ISOLATED DURING LONG-TERM TREATMENT OF URINARY-TRACT INFECTION WITH CO-TRIMOXAZOLE

Thirty out-patients treated for chronic urinary tract infection with long-term low-dosage co-trimoxazole over a period of five years were monitored for the emergence of trimethoprim resistance. Plasmids conferring high-level resistance to trimethoprim were isolated from 5.1% of the isolates examined (five patients) and belonged to the P incompatibility group and the I incompatibility complex. The W group plasmid previously isolated from several London hospitals was not detected. Significant (8 μ/ml or more) resistance to trimethoprim, presumably resulting from chromosomal mutation, was detected in a further 16.7% of strains examined (12 patients). These strains did not require thymine for growth and may represent an important resistance mechanism in patients treated with long-term co-trimoxazole. © 1979, by The British Society for Antimicrobial Chemotherapy.