Genetic analysis of multi-drug resistance and the clonal dissemination of β-lactam resistance in Salmonella Infantis isolated from broilers

Abstract: An epidemiologic study was conducted to investigate the incidence and characterize the antimicrobial resistance determinants, analyzing plasmid profiles, and establishing the genetic relationship among β-lactam-resistant isolates of Salmonella Infantis from broilers in Southern Japan. A total of 120 isolates were recovered from 56 flocks belonging to 44 holdings during 2004–2006. The percentages of resistance were as follows: ampicillin (24%), cephalothin (23%), cefoxitin (0%), ceftazidime (11%), cefotaxime (11%), chloramphenicol (0%), kanamycin (7.5%), ofloxacin (20%), oxytetracycline, streptomycin and sulfamethoxazole (100%) and trimethoprim (75%). The incidence of bla TEM-encoded β-lactam resistance in 2004–2006 was significantly higher than in 1998–2003 (P <0.001). BlnI-digested PFGE patterns generated two related clusters implicated in the dissemination of β-lactam resistance. Two types of plasmid profiles were observed and two plasmids of ca. 50 and 180-kb size were carried by β-lactam-resistant isolates. Streptomycin resistance was conferred by aadA1 (n =116), aadA1-aadA2 (n =1), and aadA1-strA-strB (n =3). Resistances to kanamycin, oxytetracycline, sulfamethoxazole and trimethoprim were conferred by aphA1 (n =9, 100%), tetA (n =120, 100%) sul1 (n =120, 100%) and dfrA5 (n =90, 100%), respectively. Two types of class 1 integrons were detected: 1.0kb (n =120) and, 1.0/1.5kb (n =3). Integrons of 1.0/1.5kb were found in isolates with the aadA1-strA-strB gene combination. For the first time, all S. Infantis isolates showed resistance to at least three classes of antimicrobial agents; and the intestinal tract of healthy poultry was a reservoir of the extended-spectrum cephalosporin-resistant isolates of serovar Infantis. [Copyright &y& Elsevier]