Occurrence and characteristics of class 1, 2 and 3 integrons in Escherichia coli, Salmonella and Campylobacter spp. in the Netherlands

Objectives: To determine the occurrence and transmission of class 1, 2 and 3 integrons in multidrug-resistant or sulfamethoxazole-resistant Salmonella from human and animal sources and in Campylobacter spp. and Escherichia coli from broilers isolated in the Netherlands in 2004. Methods: PCR, restriction fragment length polymorphism (RFLP) and DNA sequencing were used to detect integrase genes and gene cassettes within 234 E. coli isolates, 40 Campylobacter isolates and 228 Salmonella isolates. Results: Class 1 integrons were found in 76% of the E. coli and in 43% of the Salmonella isolates. Class 2 integrons were found in 11% of the E. coli and 1% of the Salmonella isolates. No class 1 or 2 integrons were detected in the Campylobacter isolates, and no class 3 integrons were detected in any of the bacterial species examined. The 22 different integrons detected harboured 20 different gene cassettes. The cassette arrays dfrA1-aadAl and dfrA1-sat2-aadAl were most frequently associated with class 1 and 2 integrons, respectively. For the first time linF was found to be associated with a class 2 integron as part of the linF-sat2-aadAl cassette. The gene cassettes found within the integrons explain only a part of the resistance profile of the isolates. Conjugation experiments demonstrated transfer of class 1 and 2 integrons. Conclusions: Our data demonstrate the importance of integrons for the occurrence and transmission of multidrug resistance. Identical predominant class 1 and 2 integrons in E. coli and Salmonella serovars indicate horizontal transfer between these species.