Stool carriage of CTX-M/CMY-producing Salmonella enterica in a Chinese tertiary hospital in Shenzhen, China

Salmonellosis, caused by non-typhoidal <i>Salmonella</i>, is a common foodborne gastrointestinal infection. Third-generation cephalosporins are recommended as the first-line treatment for <i>Salmonella</i> infections. Our study aimed to investigate the molecular epidemiology, antimicrobial resistance, and the transmission of extended-spectrum β-lactamases (ESBL) genes in 96 clinical <i>Salmonella</i> isolates collected between 2020 and 2022 at a tertiary hospital in Shenzhen, China. We performed antimicrobial susceptibility testing and whole-genome sequencing to identify serotypes, multilocus sequence typing, antimicrobial resistance genes in these isolates, and the genetic structures of the <i>bla</i> _CTX-M/<i>bla</i> _CMY genes. Seventeen <i>Salmonella</i> serotypes were identified, with <i>S</i>. 4,[5],12:i:- (37.5%) being the most common, followed by <i>S</i>. Enteritidis (15.63%), <i>S</i>. Typhimurium (14.58%), <i>S.</i> London (7.29%), and <i>S.</i> Rissen (5.21%). MLST analysis revealed 19 distinct sequence types (STs), with ST34 being the most prevalent (36.46%), followed by ST11 (15.63%) and ST19 (13.54%). Antimicrobial resistance testing showed those isolates had high levels of resistance to ampicillin (72.92%) and tetracycline (71.88%), with 70.83% of isolates as multidrug-resistant (MDR). Three <i>bla</i> _CTX-M genes (<i>bla</i> _CTX-M-14, <i>bla</i> _CTX-M-55, and <i>bla</i> _CTX-M-65) and <i>bla</i> _CMY-2 were identified among 18 cefotaxime-resistant strains, of which one and 12 isolates successfully transferred <i>bla</i> _CMY or <i>bla</i> _CTX-M to <i>E. coli</i> C600 via conjugation, respectively. The <i>bla</i> _CTX-M/<i>bla</i> _CMY-2-carrying contigs in nine <i>Salmonella</i> isolates ranged from 2,156 to 164,862 bp, were located either on the chromosome (n=1) or plasmids (IncI1, IncK1, IncA/C) (n=9), and the <i>bla</i> _CTX-M/<i>bla</i> _CMY-2 genes were associated with IS<i>Ecp1</i>. Our study demonstrates the diversity of MDR <i>Salmonella</i> serotypes in clinical isolates, and highlights the role of plasmids and mobile genetic elements in the horizontal transfer of <i>bla</i> _CTX-M/<i>bla</i> _CMY, emphasizing the need for continuous surveillance of <i>Salmonella</i> in clinical samples.