Characteristics of cephalosporin-resistant Salmonella isolates from poultry in Korea, 2010-2017

Cephalosporins are the key drugs for the treatment of salmonellosis. Resistance to cephalosporins in Salmonella spp. has become a serious public health concern worldwide. Although the sales of cephalosporins have increased by five times from 2008 to 2016 in Korea, limited information is available on cephalosporin resistance in Korea. Therefore, we aimed to investigate the trends in prevalence and characteristics of cephalosporin-resistant Salmonella isolates from poultry in Korea in the period between 2010 and 2017. A total of 141 Salmonella isolates were collected from various poultry industry sources, and 38 (27.0%) among them showed resistance to cephalosporins. In particular, resistance to the following cephalosporins increased significantly over the seven-year period cephalothin (from 5.0% to 29.2%), cephalexin, cefuroxime, cefotaxime, and ceftazidime (from 0% to 25.0%), and cefepime (from 0% to 12.5%). In addition, 12 isolates carried a beta-lactamase gene. A non-extended-spectrum beta-lactamase (ESBL) and plasmid-mediated AmpC beta-lactamase (pAmpC) gene, bla(TEM-M-79) was found in three isolates in the periods of 2010 to 2011 and 2012 to 2013, respectively. The bla(CTX-M-70) (n = 4) and bla(CTX-M-15) (n = 1) for ESBL genes and bla(CMY)(-2) (n = 1) for pAmpC genes were only present in the 2016-2017 period. All ESBL/pAmpCpositive isolates had high minimum inhibitory concentrations for most cephalosporins and showed multi drug resistance. In a conjugation experiment, the transfer of bla(CTX-M-79) and bla(CMY-2) genes was confirmed in transconjugants, which showed similar pattern of antibiotic resistance. This demonstrates that ESBL/pAmpC-producing Salmonella isolates might be transmitted to humans through contaminated poultry products. These findings suggest the need for the development of monitoring program and the guidelines for prudent use of antimicrobial agents in the poultry industry in Korea.