High-level tetracycline-resistant Neisseria gonorrhoeae in Ontario, Canada--investigation of a cluster of isolates, showing chromosomally mediated resistance to penicillin combined with plasmid-mediated resistance to tetracycline.

Between 1991 and 1994, plasmid-mediated, tetracycline-resistant Neisseria gonorrhoeae (TRNG) increased from 61.8% to 85.96% of all resistant isolates in Ontario, Canada. Ninety-nine isolates with tetracycline MICs >32 mg/L were characterized by auxotype/serovar (A/S) class, plasmid profile, hybridization with eight tetracycline-resistant probes, and pulsed-field gel electrophoresis (PFGE) of genomic DNA after digestion with NheI and SpeI restriction endonucleases. A cluster of 82 isolates with penicillin MICs of 2-4 mg/L and tetracycline MICs of 128 mg/L (chromosomally mediated resistance) belonged to A/S class NR/IB-1 and had identical or closely related PFGE profiles. Seventeen isolates, TRNG (10) and penicillinase-producing TRNG (7), with tetracycline MICs of 64-256 mg/L, belonged to eight A/S classes and displayed 12 different PFGE profiles. The 99 isolates hybridized only with the TetM probe. Phenotypic and molecular characterization indicated a diverse population throughout the Province of Ontario.