Genetic Characterization of Fluoroquinolone Resistance in Salmonella enterica Serovar Gallinarum Isolates from Chicken in Korea.

Salmonella enterica serovar Gallinarum is a nonmotile host-adapted Salmonella that causes fowl typhoid (FT), and an outbreak of FT is characterized by anorexia, greenish-yellow diarrhea, paleness, and sudden death with high mortality in poultry. To control and treat FT in commercial chickens, fluoroquinolones are widely used in Korea. This study aimed to investigate the genetic characteristics of fluoroquinolone-resistant Salmonella Gallinarum isolates from 2014-18 from chicken in Korea. A total of 35 ciprofloxacin (CIP)-resistant Salmonella Gallinarum was tested, and 22 (62.9%) isolates were observed to have multidrug resistance. All isolates had a mutation at the Ser83 or Asp87 codon in the gyrA gene, whereas three isolates had only double mutations at Ser83 → Phe and Asp87 → Asn or Ser83 → Phe and Asp87 → Gly. Minimum inhibitory concentrations of isolates with double mutations were relatively higher (≥8 mg/L for CIP and ≥16 mg/L for enrofloxacin) than those of other isolates with a single mutation in gyrA. Among 35 CIP-resistant Salmonella Gallinarum, plasmid-mediated quinolone resistance genes were detected in six (17.1%) isolates, and qnrB and qnrS were detected in four and two isolates, respectively. In the distribution of antimicrobial resistance genes in 35 CIP-resistant Salmonella Gallinarum, ant(2″)-I (54.3%) was the most prevalent gene, followed by TEM-1 (14.3%), sul1 (11.4%), and cmlA (5.7%). Fifteen (42.9%) of the 35 CIP-resistant Salmonella Gallinarum also carried class 1 integrons, which showed five types of resistance gene cassettes: aadA2 (7 isolates), aadA2 + dfrA12 (5 isolates), and aadA1 + aad A2 (3 isolates). Among plasmid replicons, 23 isolates (65.7%) carried five different plasmid replicons: Frep (9 isolates), FIB (7 isolates), FIIA (6 isolates), B/O (4 isolates), and I1 (3 isolates). These results suggest that continued monitoring of fluoroquinolone resistance is necessary to preserve the effectiveness of fluoroquinolones in poultry and to surveil the transmission to humans through the food chain.