The genetic background for streptomycin resistance in Escherichia coli influences the distribution of MICs

Objectives: The aim of this study was to investigate the genetic background for streptomycin resistance in Escherichia coli and perform analysis of the MICs in relation to genetic background. Methods: The 136 strains investigated, with streptomycin MICs of >= 16 mg/L, originated from meat and meat products and were collected within the frame of the Norwegian monitoring programme for antimicrobial resistance in bacteria from feed, food and animals (NORM-VET). PCR was carried out for detection of the streptomycin resistance genes strA-strB and the integron-associated aadA gene cassettes. Results: The strA-strB genes and/or an aadA gene cassette were detected in 110 of the 136 (80.9%) strains investigated. The strA-strB genes were the most prevalent, and were detected in 90 strains. The aadA gene cassettes were detected in 29 strains, and nine strains harboured both the strA-strB genes and an aadA gene cassette. The distribution of MICs differed considerably between isolates harbouring the strA-strB genes (solely) (MIC50=128 mg/L) and isolates harbouring an aadA gene cassette (solely) (MIC50=16 mg/L). Strains harbouring both the strA-strB genes and an aadA gene cassette had higher streptomycin MICs than those harbouring either alone. Conclusions: The distribution of streptomycin MICs in E. coli can be greatly influenced by the genes encoding resistance to streptomycin. The strA-strB genes are probably involved in conferring high-level resistance to streptomycin, whereas the opposite seems to be the case for the aadA gene cassettes. The low-level streptomycin resistance, caused by the presence of aadA gene cassettes in integrons, represents an obstacle in classifying E. coli as susceptible or resistant to streptomycin. Furthermore, the determination of an epidemiological cut-off value for surveillance purposes is also complicated by dissemination of integrons containing the aadA cassettes.