Molecular characteristic of antimicrobial resistance of Salmonella Gallinarum isolates from chickens in Korea, 2014 to 2018

ABSTRACT Fowl typhoid (FT), which is caused by Salmonella enterica serovar Gallinarum (S. Gallinarum), leads to high morbidity and acute or subacute mortality in chickens of all ages. Although a live S. Gallinarum 9R vaccine was introduced in 2001 for commercial layer chickens in Korea, until recently, a variety of antimicrobials were widely used to prevent or treat FT. In this study, we investigated antimicrobial resistance in S. Gallinarum strains isolated from 2014 to 2018 and characterized the multidrug-resistant (MDR) strains to better understand the resistance trends in recent isolates. A total of 130 S. Gallinarum isolates were collected from chickens with FT, and the isolates showed highest rates of resistance to nalidixic acid (78.5%), followed by gentamicin (52.3%), ciprofloxacin (26.9%), and ampicillin (14.6%). Particularly, significant increases (P The distribution of antimicrobial resistance genes in the 39 MDR S. Gallinarum isolates was as follows: ant(2”)-I gene (22 isolates), blaTEM-1 gene (13 isolates), sul1 (9 isolates), sul2 (3 isolates), cmlA (3 isolates), and qnrB (3 isolates). Of 39, 25 (64.1%) MDR S. Gallinarum isolates also carried class 1 integrons, and these showed 5 types of resistance gene cassettes: dfr A12+aad A2 (36.0%), aad A2 (36.0%), aad A1-aad A2 (20.0%), dfr A12+cat B3+aad A2 (4.0%), and dfr A12 (4.0%). Among the plasmid replicons, B/O (33.3%) was more prevalent than the other replicon types, followed by Frep (25.0%), FIIA (19.4%), FIB (13.9%), and I1 (8.3%). Antimicrobial resistance may become a serious problem because many drugs are likely ineffective for the treatment of FT. Therefore, these data support the critical need for comprehensive surveillance of antimicrobial resistance in poultry.