Nalidixic acid-a good marker of fluoroquinolone resistance mechanisms in Escherichia coli

Abstract

<p>The purpose of this study was to evaluate how ciprofloxacin, pefloxacin, and nalidixic acid disks perform in screening fluoroquinolone resistance mechanisms in 278 <i>Escherichia col</i>i isolates collected from a prospective clinical material. Antimicrobial susceptibility testing of ciprofloxacin, pefloxacin, and nalidixic acid was performed with the disk diffusion method. PCR-based and sequencing methods were used to detect chromosomal mutations in the<i> gyr</i>A and <i>parC</i> genes and the presence of plasmid-mediated qnr and <i>aac(6 ')-1b-cr</i> genes. In addition, whole-genome sequencing was used to confirm these results. Our results show that fluoroquinolone resistance mechanisms were discovered, even in ciprofloxacin-susceptible isolates, and plasmid-mediated low-level fluoroquinolone resistance is easily missed if only ciprofloxacin disk is used. <i>E. coli</i> strains with chromosomal <i>gyrA</i> and/or<i> par</i>C mutations were well detected with pefloxacin disk. However, nalidixic acid was a superior tool to detect and differentiate between low- (plasmid-mediated) and high-level (chromosomal mutations) fluoroquinolone resistance in <i>E. col</i>i. Thus, more clinical studies are needed to evaluate the clinical relevance of fluoroquinolone resistance mechanisms in enteric bacteria and pathogens that show potential but are not yet phenotypically fluoroquinolone-resistant.<br></p><p>IMPORTANCEWe show in our clinical setting that fluoroquinolone resistance mechanisms are discovered, even among phenotypically fluoroquinolone-susceptible Escherichia coli isolates. When plasmid-mediated quinolone-resistance determinants are present, they are a potential risk for treatment failures due to accumulation of resistance mechanisms during the antimicrobial treatment. Therefore, when it is clinically relevant, fluoroquinolone resistance mechanisms in <i>E. col</i>i should be monitored more closely, and we also recommend testing nalidixic acid susceptibility.<br></p>