Trimethoprim resistance in enterobacteria isolated in Chile.

Four hundred strains of enterobacteria, isolated before the introduction of trimethoprim as a single agent in Chile, were studied. One hundred and thirty six strains (34%) were resistant to greater than or equal to 8 mg/l of trimethoprim and 118 of these (87%) were also resistant to sulphamethoxazole (156 mg/l). Plasmids were detected in 79% of the trimethoprim-resistant strains; 56% of the strains carried two or more plasmids. Trimethoprim resistance could be transferred by conjugation in 66% of the plasmid-carrying strains; this resistance was co-transferred with a variety of other antibiotic resistance determinants.