Susceptibility testing of Bordetella pertussis.

Korgenski and Daly recommend charcoal horse blood agar (Regan-Lowe agar) for the detection of erythromycin resistance in Bordetella pertussis by agar dilution, Etest methodology, or agar diffusion. MICs for most isolates were #0.06 to 0.12 mg/ml by agar dilution and the Etest, while the MIC for the only resistant strain was 16 to 64 mg/ml (4). Charcoal agar may be suitable for discrimination between erythromycin susceptibility and resistance in B. pertussis, but I would like to caution against the use of charcoal agar for susceptibility testing of B. pertussis in general. A previous study by our group showed that agar dilution MICs of erythromycin for B. pertussis were several dilutions higher on charcoal agar than those determined on Mueller-Hinton agar and that the MIC for Staphylococcus aureus ATCC 29213 on charcoal agar exceeded the recommended reference range (2). Similar results were obtained with antimicrobial agents other than erythromycin (3). A number of studies relating to Legionella susceptibility testing on the charcoal-containing BCYE-a agar have stressed the inactivation by charcoal of many antimicrobial agents, including erythromycin (1, 5, 6). Mueller-Hinton agar supplemented with 5% horse blood is the most suitable medium for susceptibility testing of B. pertussis by agar dilution (2). Appropriate media for the Etest and the role of the agar diffusion method for testing a slow-growing fastidious organism like B. pertussis should be evaluated in larger studies with different antimicrobial agents.