Objectives: The association of trimethoprim-resistant dfr genes with integrons was investigated in urinary Escherichia coli isolates in Korea from the last two decades.
Methods: Of 623 E. coli isolates from urine specimens, 421 trimethoprim-resistant isolates were studied for dfr genes associated with integrons. Integrase genes were amplified and the PCR products restricted using HinfI to classify integron types. Gene cassette regions for the class 1 and class 2 integrons were amplified and sequenced. PFGE was performed to determine the epidemiological relationship of E. coli isolates.
Results: The carriage of class 1 integrons was found to be significantly higher in trimethoprim-resistant isolates (69%) than in trimethoprim-susceptible isolates (19%). Among the trimethoprim-resistant isolates, the frequency of dfr genes associated with class 1 integrons increased sharply from 10% of the isolates during 1980-1985 to 53% during 1996-1997 and to 46% during 2001-2002. Five different dfr cassettes--dfrA1, dfrA5, dfrA7, dfrA12 and dfrA17--were identified among the urinary E. coli isolates from the last two decades; dfrA12 was the most prevalent during 1980-1985 and dfrA17 during 1996-1997 and 2001-2002. The majority of dfr genes associated with class 1 integrons were conjugally transferable to recipient E. coli strains. The E. coli isolates that carried dfrA17 associated with class 1 integrons were found to be phylogenetically unrelated, indicating that dfrA17 was widely distributed in the different clones of E. coli.
Conclusion: Class 1 integrons were found to be an important genetic element of resistance to trimethoprim among urinary E. coli in Korea, and the prevalence of dfrA17 was mainly due to the horizontal transfer of class 1 integrons through conjugative plasmids.