Transferable high-level trimethoprim resistance among isolates of Escherichia coli from urinary tract infections in Ontario, Canada.

Of 1171 isolates of Escherichia coli isolated from urine samples at the Public Health Laboratory, Toronto, Ontario, Canada, between May 1990 and December 1991, 120 (10.3%) were resistant to trimethoprim (TMP), cotrimoxazole (TMP/SMX), sulfamethoxazole (SMX) and other antimicrobial agents; 110 of the 120 isolates (91.7%) were resistant to four or more agents. The majority of resistant isolates (91.7%) exhibited high-level resistance (MIC > 1000 mg/L) to TMP. The MIC of TMP/SMX for all 120 isolates was > 2.0/38.0 mg/L and for SMX > 1024 mg/L. High-level resistances were also present among the beta-lactam antimicrobials with MICs ranging from 16- > 256 mg/L. Forty-three of 120 TMP-resistant (35.8%) isolates conjugally transferred TMP-resistance to E. coli K-12. Co-transfer of several other resistances was observed. SMX cotransferred from 86% of the 43 donors and beta-lactams together with SMX cotransferred from 70%. Nalidixic acid resistance was present among 22 (18.3%) of the 120 resistant isolates, however, nalidixic acid resistance was not transferred to E. coli K-12.