Antimicrobial resistances of extraintestinal pathogenic Escherichia coli isolates from swine in China

Antibiograms and relevant genotypes of porcine extraintestinal pathogenic Escherichia coli (ExPEC) isolates (n = 315) recovered between 2004 and 2007 in China were assessed. Among the 14 antimicrobials tested, the most prevalent resistance was to ampicillin, trimethoprim, sulfadimidine, tetracycline, neomycin, streptomycin, kanamycin, ciprofloxacin and ofloxacin (ranging from 81.9 to 100%). Forty-six multiresistant patterns were found. For each antimicrobial agent, ampicillin resistance was primarily mediated by bla(TEM,) streptomycin resistance by strA and strB, kanamycin/neomycin resistance by aphA1, gentamicin resistance by aac(3)-IV, quinolones resistance by mutations in gyrA, tetracycline resistance by tet(A), tet(B) and tet(G), trimethoprim resistance by dfrA7, dfrA12 and dfrA13, and sulfadimidine resistance by sul1 and sul2. Both bla(TEM-1) and bla(CTX-M-14) were found in two ESBLs-producing isolates. Strains that harbored several genes that conferred resistance to the same antimicrobial agent were often significantly more multiresistant than others. Class 1 integrons were identified in 86 (27.3%) ExPEC isolates, which harbored dfrA14, aadA2, aadA22, dfrA17, aadA5, dfrA17-aadA2, dfrA1-aadA1, dfrA12-aadA2, dfrA17-aadA5 gene cassettes in five major different variable regions, conferring resistance to trimethoprim and aminoglycosides. These results provide novel insights into the epidemiological characteristics of porcine ExPEC strains in China, and suggest the need for the prudent use of antimicrobial agents in food animals.