Class 1 integrons and resistance gene cassettes among multidrug resistant Salmonella serovars isolated from slaughter animals and foods of animal origin in Ethiopia

The present study was undertaken to identify and characterize integrons and integrated resistance gene cassettes among multidrug resistant (MDR) Salmonella isolates from slaughter animals and food products of animal origin in Ethiopia. A total of 98 epidemiologically unrelated Salmonella isolates comprising 13 serovars were characterized using serotyping, phage typing, antimicrobial resistance testing and the pulsed-field gel electrophoresis (PFGE) method. Integron-PCR was used to detect the presence of class 1 and class 2 integrons in the MDR strains. The associated individual resistance gene cassettes were identified using specific PCRs and DNA sequencing. The location of the integrons was determined by Southern blot hybridization analysis. Among the Salmonella serovars, a high level of antimicrobial resistance was found to streptomycin (82.6%), tetracycline (75.5%), sulfamethoxazole (60.2%), spectinomycin (53.1%), ampicillin (42.8%), nalidixic acid (34.7%), nitrofurantoin (30.6%), trimethoprim (27.5%), gentamicin (20.4%) and ciprofloxacin (19.4%). Class 1 integrons were detected in 53.1% of the MDR isolates comprising serovars Anatum, Braenderup, Kentucky, Saintpaul and Typhimurium. Of the class 1 integron positive isolates 61.5% harboured the integron-associated gene cassettes: aadA2, aadA2+bla(PSE-1), dfrA1-aadA1 and dfrA12-orf-aadA2 (amplicon sizes 1000 bp, 1000+1200 bp, 1600 bp and 1900 bp, respectively). The chromosomally located aadA2 and aadA2+bla(PSE-1) resistance gene cassettes occurred exclusively in S. Typhimurium DT104 isolates, the other cassettes were found on large plasmids in several serovars. An aacCA5-aadA7 gene cassette array (amplicon size 1600 bp) was exclusively found in all MDR S. Kentucky strains of R type Str/SpeSmxGenNalAmpTetCipCef and this integron was shown to be chromosomally located. Results of the present study indicate that class 1 integrons carrying gene cassettes, which confer resistance to different classes of antimicrobials such as aminoglycosides, beta-lactams and trimethoprim are widespread among the MDR Salmonella serovars isolated from slaughter animals and food products of animal origin in Ethiopia indicating the important role of these genetic elements in the dissemination of multidrug resistance.