Comparison of automated microbroth dilution and agar dilution for antimicrobial susceptibility of Campylobacter jejuni isolated from dairy sources

Objectives: To compare the agreement between microbroth dilution and agar dilution for antimicrobial susceptibility testing of Campylobacter jejuni. Methods: Utilizing commercially prepared antimicrobial panels, microbroth dilution was compared with agar dilution for determining antimicrobial susceptibility in C. jejuni isolates. To assess the performance of both techniques for ampicillin, 190 C. jejuni isolates from dairy cattle were utilized. A group of 172 C. jejuni isolates from dairy sources were used to compare the susceptibility to ciprofloxacin, erythromycin, nalidixic acid and tetracycline. Results: Our results indicate that microbroth dilution and agar dilution agree within ±1 log₂ dilution for 86.7% of the isolates tested. Ciprofloxacin had the highest level of agreement for isolates tested by both techniques, resulting in a kappa of 0.886 and 97.1% agreement ±1 log₂ dilution. The least agreement was observed in determining the susceptibility of isolates to ampicillin and erythromycin (82.1 and 79.7% agreement ±1 log₂ dilution). However, kappa statistics were considered to have good agreement for these antimicrobials. There were no significant differences in the summary statistics for any of the five antimicrobials evaluated for the isolates analysed by the percentage of resistant isolates, MIC₅₀, MIC₇₅ or MIC₉₀ beyond ±1 log₂ dilution. There was no association in the classification of resistance by the testing methods employed. We also demonstrated that the quality control strain of C. jejuni ATCC 33650 performed in a consistent manner for both agar dilution and microbroth dilution. Conclusions: Microbroth dilution may be an acceptable alternative to agar dilution for determining susceptibility of C. jejuni in research or surveillance where flow of samples, labour efficiency and cost may restrict the use of agar dilution.