Comparison of the Etest and agar dilution for in vitro antimicrobial susceptibility testing of Campylobacter

Abstract

The performance of the Etest and agar dilution for in vitro antimicrobial susceptibility of Campylobacter spp. was evaluated using a quality control strain Campylobactor jejuni ATCC 33560, and 81 C. jejuni and 54 Campylobacter coli isolates recovered from retail raw meats. Seven antimicrobial agents: chloramphenicol, ciprofloxacin, doxycycline, erythromycin, gentamicin, nalidixic acid and tetracycline, were tested using the two methods, whereas azithromycin was tested using the Etest only. The correlation between the Etest and agar dilution MICs varied greatly depending on the antimicrobial agents tested. The overall agreement of MICs (+/-1 log(2) dilution) between the two methods was 61.9%, ranging from 21.4% for nalidixic acid to 92.6% for gentamicin. MICs obtained using the Etest were generally lower than those by agar dilution regardless of the species of organism tested. MIC(50) and/or MIC(90) values were at least one dilution lower for the Etest than for agar dilution when testing chloramphenicol, ciprofloxacin, doxycycline, erythromycin and nalidixic acid. Based on the agar dilution MICs, the resistant rate of the 135 Campylobacter isolates was highest for tetracycline (82.2%), followed by doxycycline (78.5%), nalidixic acid (21.5%), ciprofloxacin (20.7%) and erythromycin (17.0%). None of the isolates demonstrated resistance to chloramphenicol or gentamicin. The study indicated that the Etest results were not in complete agreement with the agar dilution test. Although the Etest has been proven to be a satisfactory testing method, its use for Campylobacter susceptibility testing requires further standardization. The study also showed that C. jejuni and C. coli isolates resistant to antimicrobials used for treating campylobacteriosis were common in retail raw meats.

Prevalence and antibiotic resistance pattern of Campylobacter species in foods of animal origin

“…In the present study, most of the isolates were found to be sensitive to the fluoroquinolone class of antibiotics, the only exception being ciprofloxacin to which 26% of the isolates showed resistance. The similar findings for Campylobacter isolate from foods of animal origin were reported by Ge et al [18]. The resistance to erythromycin and gentamicin was reported to be 0% and 10%, respectively.…”

Campylobacter

“…This observation was particularly evident with respect to C. jejuni [84]. Comparing MIC values obtained by E-test and the agar dilution protocols, Ge et al [48] reported values ranging from 21.4 to 62% for gentamicin and nalidixic acid respectively. Whilst the E-test is convenient, relatively simple to perform, MIC values determined by this method are lower when compared to similar values obtained by the agar dilution method regardless of the organism tested [41,48,60].…”

“…Several laboratory methods, including disc diffusion, broth microdilution, agar dilution and the Epsilometer-test (E-test) have been applied to determine in vitro susceptibility profile(s) of Campylobacter to a range of antimicrobial agents [15,41,45,46,48,84,119,149]. Despite the availability of comparable standardised procedures for many organisms, based on, the approved guidelines defined by the National Committee for Clinical Laboratory Standards (NCCLS), no internationally accepted criteria for susceptibility testing of Campylobacter spp.…”

“…Values determined however can vary depending on the antimicrobial agent(s) being considered [48]. This observation was particularly evident with respect to C. jejuni [84].…”

“…Comparing MIC values obtained by E-test and the agar dilution protocols, Ge et al [48] reported values ranging from 21.4 to 62% for gentamicin and nalidixic acid respectively. Whilst the E-test is convenient, relatively simple to perform, MIC values determined by this method are lower when compared to similar values obtained by the agar dilution method regardless of the organism tested [41,48,60]. However when E-test and the agar dilution method are used on a small number of isolates from a single geographic location, acceptable agreement between both approaches for susceptibility categorisation is achieved.…”