Molecular Characterization of Cephalosporin and Fluoroquinolone Resistant Salmonella Choleraesuis Isolated from Patients with Systemic Salmonellosis in Thailand

Abstract

The antimicrobial resistance of nontyphoidal <i>Salmonella</i> has become a major clinical and public health problem. Southeast Asia has a high level of multidrug-resistant <i>Salmonella</i> and isolates resistant to both fluoroquinolone and third-generation cephalosporins. The incidence of co-resistance to both drug classes is a serious therapeutic problem in Thailand. The aim of this study was to determine the antimicrobial resistance patterns, antimicrobial resistance genes and genotypic relatedness of third-generation cephalosporins and/or fluoroquinolone-resistant <i>Salmonella</i> Choleraesuis isolated from patients with systemic salmonellosis in Thailand. Antimicrobial susceptibility testing was performed using the agar disk diffusion method, and ESBL production was detected by the combination disc method. A molecular evaluation of <i>S.</i> Choleraesuis isolates was performed using PCR and DNA sequencing. Then, a genotypic relatedness study of <i>S.</i> Choleraesuis was performed by pulse field gel electrophoresis. All 62 cefotaxime-resistant <i>S.</i> Choleraesuis isolates obtained from 61 clinical specimens were multidrug resistant. Forty-four isolates (44/62, 71.0%) were positive for ESBL phenotypes. Based on the PCR sequencing, 21, 1, 13, 23, 20 and 6 ESBL-producing isolates harboured the ESBL genes <i>bla</i>_CTX-M-14, <i>bla</i>_CTX-M-15, <i>bla</i>_CTX-M-55, <i>bla</i>_CMY-2, <i>bla</i>_ACC-1 and <i>bla</i>_TEM-1, respectively. This study also found that nine (9/62, 14.5%) isolates exhibited co-resistance to ciprofloxacin and cefotaxime. All of the co-resistant isolates harboured at least one PMQR gene. The <i>qnr</i> genes and the <i>aac(6')-Ib-cr</i> gene were the most prevalent genes detected. The QRDR mutation, including the <i>gyrA</i> (D87Y and D87G) and <i>parC</i> (T57S) genes, was also detected. PFGE patterns revealed a high degree of clonal diversity among the ESBL-producing isolates.