Molecular characterization of the antimicrobial resistance of Riemerella anatipestifer isolated from ducks.

The antimicrobial susceptibility of 103 Riemerella anatipestifer isolates obtained from ducks during 2008 and 2010 in Southern China, to 23 antimicrobial agents was investigated using the agar dilution method. The MIC(50) and MIC(90) values of streptomycin, kanamycin, gentamicin, apramycin, amikacin, neomycin, nalidixic acid and sulfadimidine were high (32-≥ 128 μg/ml) among the 103 R. anatipestifer isolates. However, relatively low MIC(90) values (8 μg/ml) of ampicillin and florfenicol were observed among these isolates. The presence of resistance genes and integrons was determined using PCR. The genes bla(TEM-1), aph(3')-VII, aadA1, aadA2, aac(3')-IV, aac(3')-IIc, aac(6')-Ib, cat2, cmlA, floR, tet(A), tet(B), tet(C), sul1, and sul2 were detected in 1, 2, 24, 35, 11, 4, 67, 16, 26, 10, 6, 1, 9, 36 and 2 isolates, respectively. Twenty isolates contained one or two class 1 integrons carrying aadA2 or aac(6')-II-catB3-aadA1 gene cassette(s). Mutation analysis of the quinolone resistance-determining regions (QRDRs) of 43 R. anatipestifer isolates with nalidixic acid MICs ≥ 32 μg/ml, showed that the most prevalent mutations in gyrA were those resulting in the amino acid exchanges Ser83-Ile (n=37), followed by Asp87-His (n=7) and Ser83-Arg (n=5). Point mutations in parC (Arg120-Glu) were observed in 5 isolates with a ciprofloxacin MIC of >16 μg/ml. No plasmid-mediated quinolone resistance gene was detected. PFGE analysis showed that the clonal spread of multi-drug resistant R. anatipestifer isolates occurred in the same farm or between different farms. Our results reported, for the first time, the mechanism of quinolone resistance in R. anatipestifer.