The phenotypic and molecular characteristics of antimicrobial resistance of Salmonella enterica subsp. enterica serovar Typhimurium in Henan Province, China

Background: Salmonella enterica subsp. enterica serovar Typhimurium infections continue to be a significant public health threat worldwide. The aim of this study was to investigate antibiotic resistance among 147 S. Typhimurium isolates collected from patients in Henan, China from 2006 to 2015. Methods: 147 S. Typhimurium isolates were collected from March 2006 to November 2015 in Henan Province, China. Antimicrobial susceptibility testing was performed, and the resistant genes of ciprofloxacin, cephalosporins (ceftriaxone and cefoxitin) and azithromycin were detected and sequenced. Clonal relationships were assessed by multilocus sequence typing (MLST) and pulsed-field gel electrophoresis (PFGE). Results: Of the 147 isolates, 91.1% were multidrug resistant (MDR), with 4.1% being resistant to all antibiotic classes tested. Of concern, 13 MDR isolates were co-resistant to the first-line treatments cephalosporins and ciprofloxacin, while three were also resistant to azithromycin. Seven PFGE patterns were identified among the 13 isolates. All of the isolates could be assigned to one of four main groups, with a similarity value of 89%. MLST assigned the 147 isolates into five STs, including two dominant STs (ST19 and ST34). Of the 43 ciprofloxacin-resistant isolates, 39 carried double gyrA mutations (Ser83Phe, Asp87Asn/Tyr/Gly) and a single parC (Ser80Arg) mutation, including 1 isolate with four mutations (gyrA: Ser83Phe, Asp87Gly; parC: Ser80Arg; parE: Ser458Pro). In addition, 12 isolates not only carried mutations in gyrA and parC but also had at least one plasmid-mediated quinolone resistance (PMQR) gene. Among the 32 cephalosporin-resistant isolates, the most common extended-spectrum β-lactamase (ESBL) gene was bla OXA-1, followed by bla CTX-M, bla TEM-1, and bla CMY-2. Moreover, the mphA gene was identified in 5 of the 15 azithromycin-resistant isolates. Four MDR isolates contained ESBL and PMQR genes, and one of them also carried mphA in addition. Conclusion: The high level of antibiotic resistance observed in S. Typhimurium poses a great danger to public health, so continuous surveillance of changes in antibiotic resistance is necessary.