Prevalence of Class 1 Integrons and Antibiotic Resistance Patterns in Bacteria of Swine and Chicken in the US and Thailand

Studies were conducted to investigate the prevalence and potential for transfer of class 1 integrons and antimicrobial resistance in bacteria of broiler chickens and swine from the US and Thailand. Antibiograms were characterized and integron sequences were detected using standard methods. To determine if transfer of integrons occurred between bacterial species the location of the integrons (plasmid versus chromosome) was determined, and when integron-positive E. coli and Salmonella isolates possessed identical amplicon patterns, PCR products were sequenced to determine homology. Class 1 integrons were detected in 1,732 of 3,824 isolates from broiler chickens and 1,782 of 4,253 isolates from swine. Simultaneous presence of three conserved class 1 integron genes was found in 1,044 and 215 of isolates from chickens and swine, respectively. A high proportion of bacterial isolates from chickens demonstrated resistance to tetracycline, sulfamethoxazole, cephalothin, and ampicillin. A high proportion of isolates from swine demonstrated resistance to tetracycline, sulfamethoxazole, streptomycin, and ampicillin. Nine integron amplicons, with sizes ranging from 0.5 to 2.5 kb, were found, and we discovered a single swine farm on which similar integrons were observed in both E. coli and Salmonella. Sequence analysis revealed that a 1.0 kb amplicon found in both bacterial species contained an aadA1 gene cassette encoding aminoglycosides 3’-adenyltransferase, confering resistance to streptomycin and spectinomycin. A 2.0 kb amplicon was also found in both types of bacteria containing the aadA5 gene encoding aminoglycosides 3’-adenyltransferase, an additional reading frame with unknown function, orfD, as well as a dfrA17 gene encoding dihydrofolate reductase, conferring resistance to trimethoprim. Our results indicate that class 1 integrons are common in commensal and foodborne bacteria in broiler chickens and swine, and that some, but not all antibiotic resistances are associated with the presence of class 1 integrons. Identical integrons found in Salmonella and E. coli from a single farm likely indicate transfer between these two organisms occurs via exchange of plasmids. This work provides additional knowledge regarding the complex nature of antibiotic resistance gene acquisition, reservoirs, and transfer that should aid in development of courses of action and strategies for control of these potential foodborne and zoonotic hazards.