Characterization of plasmid-mediated quinolone resistance by the qnrS gene in Escherichia coli isolated from healthy chickens and pigs

The prevalence of qnr and qepA genes in 660 Escherichia coli isolates was investigated in healthy animals from 30 pig farms and 30 chicken farms in Taiwan from January 2005 to February 2006 by the polymerase chain reaction. The qnrS gene, but not qnrA, qnrB, and qepA were detected in 12/360 pig isolates (3.33%) and in 6/300 chicken isolates (2%). Southern blot hybridization analysis indicated that qnrS was located on plasmids ranging in size from 50-165 kb. Eleven of the 18 qnrS positive isolates which showed a high ciprofloxacin resistance phenotype (minimum inhibitory concentration >= 8 mg/l) also had amino acid sequence variations in chromosomal quinolone resistance-determining regions of gyrA and parC. Only two qnrS-positive isolates carried the aac(6')-Ib-cr variant that mediates FQ acetylation. For the high percentage resistance of cephalosporins, the bla(CTX-M) gene was also examined in qnrS-positive isolates. The bla(CTX-M) gene was detected in fifteen isolates (15/18, 83.3%) of which 12 isolates were bla(CTX-M-1) and three isolates were bla(CTX-M-15). This study demonstrated a close linkage between the qnrS gene and bla(CTX-M-1), suggesting CTX-M and Qnr-based mechanisms might be co-emerging in E. coli strains isolated from healthy chickens and pigs under selective pressure of quinolone and cephalosporine administration.