Performance of Etest and Disk Diffusion for Detection of Ciprofloxacin and Levofloxacin Resistance in Salmonella enterica

Abstract

dWe compared Etest and disk diffusion to broth microdilution for the detection of fluoroquinolone resistance in 135 typhoidal and nontyphoidal serovars of Salmonella. Categorical agreements for the ciprofloxacin and levofloxacin Etests were 89.6 and 83.7%, respectively. Disk diffusion categorical agreements were 88.2 and 93.3%, respectively. Only minor errors were observed. S almonella gastroenteritis is normally a self-limiting disease, and antimicrobial therapy is not generally recommended (1, 2). However, antimicrobial therapy is indicated for the management of severe diarrhea or extraintestinal infection caused by Salmonella spp. and for the treatment of enteric fever, which is caused by Salmonella enterica serovars Typhi and Paratyphi A, B, and C. The fluoroquinolones ciprofloxacin and levofloxacin are important agents for the treatment of these infections (3). Fluoroquinolone resistance in Salmonella is a significant clinical concern, particularly among typhoid serovars of Salmonella isolated in Asia. The most common fluoroquinolone resistance mechanism among these isolates is mutation to the quinolone resistance-determining region (QRDR) of the topoisomerase gene gyrA, resulting in elevated ciprofloxacin and levofloxacin MICs (0.12 to 0.5 g/ml) and full resistance to the quinolone nalidixic acid (MIC, Ն32 g/ml) (4). Patients infected with such isolates have delayed responses to fluoroquinolone therapy and, in many cases, clinical failures (4). As such, the Clinical and Laboratory Standards Institute (CLSI) has historically instructed laboratories to perform a nalidixic acid disk diffusion or MIC test in order to screen for this resistance mechanism among Salmonella isolates for which susceptibility testing was warranted (4). However, fluoroquinolone resistance in Salmonella can also be attributed to mutation to the QRDR of other topoisomerase genes (gyrB, parC/E) or by the acquisition of plasmid-mediated quinolone resistance determinants, such as the qnr and aac(6=)-Ib-cr genes. In contrast to QRDR topoisomerase mutations, isolates with fluoroquinolone resistance associated with plasmid-mediated mechanisms are not necessarily resistant to nalidixic acid (5).In 2013, the CLSI revised the ciprofloxacin, levofloxacin, and ofloxacin MIC interpretive criteria (breakpoints) for Salmonella and the disk diffusion breakpoints for ciprofloxacin, in part to better detect these fluoroquinolone resistance mechanisms in Salmonella spp. (Table 1) (6). At present, no levofloxacin or ofloxacin disk diffusion breakpoints for Salmonella have been established by the CLSI. The U.S. Food and Drug Administration (FDA) has accordingly revised the ciprofloxacin MIC interpretive criteria for S. enterica serovar Typhi but not for other Salmonella serovars or for levofloxacin or ofloxacin. Furthermore, no commercial MIC susceptibility test panels produced in the United States contain fluoroquinolone concentrations low enough to allow use of the current CLSI breakpoints. Thus, laboratories in the United States have the option of perf...

Evaluation of Surrogate Disk Tests for Detection of Ciprofloxacin and Levofloxacin Resistance in Clinical Isolates of Salmonella enterica

“…No Salmonella levofloxacin disk breakpoints have been formally established, and yet levofloxacin is commonly used in many U.S. hospitals as the formulary fluoroquinolone. Performance of the proposed levofloxacin disk breakpoints for S. enterica is reported elsewhere (4,5). We therefore assessed the performance of ciprofloxacin, nalidixic acid, and pefloxacin disks as potential surrogate agents for the detection of levofloxacin-susceptible versus non-levofloxacin-susceptible MICs.…”

“…The European Committee on Antimicrobial Susceptibility Testing (EUCAST) independently published Salmonella-specific MIC breakpoints for ciprofloxacin, and the U.S. Food and Drug Administration (FDA) revised the ciprofloxacin MIC breakpoints to match those of the CLSI, but for S. enterica serovar Typhi alone (Table 1). CLSI established Salmonella disk diffusion breakpoints for ciprofloxacin (3), but levofloxacin and ofloxacin disk breakpoints have not yet been established, although these have been proposed by SjolundKarlsson and colleagues (4,5).…”

Commentary: Fluoroquinolone Resistance in Salmonella and the Utility of Perfloxacin Disk Diffusion

“…Laboratories able to directly measure FQ MICs are likely to find MICs to be the most straightforward approach for determining FQ susceptibility in Salmonella (Table 1). If broth microdilution is not available, the ciprofloxacin Etest is reported to show 89.6% categorical agreement with the reference broth microdilution method, with only minor errors (36).…”

Understanding and Addressing CLSI Breakpoint Revisions: a Primer for Clinical Laboratories

“…Over the course of the past several years, the CLSI has updated the fluoroquinolone breakpoints for Salmonella spp. several times (Table 1), as has been described elsewhere (10)(11)(12). Treatment of nontyphoidal Salmonella (NTS) infections limited to the gut usually consists of fluid and electrolyte replacement; antimicrobial treatment of NTS diarrhea is not required and may in fact prolong the carrier state.…”

“…Because U.S. laboratories are likely to encounter Salmonella spp. sporadically, AST can be conducted on a per-request basis, using manual methods such as ciprofloxacin disk diffusion or ciprofloxacin/levofloxacin gradient diffusion, which perform well (10,11). In the past, nalidixic acid was used as a surrogate for fluoroquinolone resistance in Salmonella.…”