Comparison of disc diffusion and agar dilution methods for antibiotic susceptibility testing of Campylobacter jejuni subsp. jejuni and Campylobacter coli

The correlation between disc diffusion and agar dilution susceptibility testing of five antibiotics was studied against 145 Campylobacter strains: 99 Campylobacter jejuni subsp. jejuni and 46 Campylobacter coli. The percentages of true results and 95% CI for disc diffusion for resistant strains were 100% (93.2-100%) for tetracycline (53 strains tested), 100% (77.2-100%) for ciprofloxacin (13 strains tested), 86.7% (62.1-96.3%) for nalidixic acid (15 strains tested), 100% (56.6-100%) for erythromycin (five strains tested) and 68.8% (44.4-85.8%) for ampicillin (16 strains tested). The percentages of true results and 95% CI were 97.6-100% and 93.2-100% respectively for 89-140 susceptible strains to the five antibiotics tested. There was a 1.4% major error for nalidixic acid, 0.7% very major error for erythromycin, 5.5% and 1.4% minor and major errors respectively for ampicillin. There was complete agreement for ciprofloxacin and tetracycline. Results of ampicillin susceptibility are not expected to be useful in a clinical setting. The nalidixic acid disc is a marker of ciprofloxacin susceptibility as the nalidixic acid-susceptible strains were susceptible to ciprofloxacin while most of the resistant ones were resistant to ciprofloxacin. Overall, our results suggest that disc diffusion is a reliable, easy and inexpensive susceptibility testing method for C. jejuni and C. coli for erythromycin, ciprofloxacin and tetracycline. Until more erythromycin- and ciprofloxacin-resistant strains are tested to confirm the reliability of this test, the resistance to these drugs needs to be confirmed using the Etest or the agar dilution method.