A simplified method for testing Bordetella pertussis for resistance to erythromycin and other antimicrobial agents

Present methods of antimicrobial susceptibility testing of Bordetella pertu ssis are time consuming and require specialized media that are not commerci ally available. We tested 52 isolates of B. pertussis for resistance to ery thromycin, trimethoprim-sulfamethoxazole, chloramphenicol, and rifampin by agar dilution with Bordet-Gengou agar (BGA) containing 20% horse blood (ref erence method), Etest using EGA and Regan-Lowe agar without cephalexin (RL- C), and disk diffusion using EGA and RL-C. The organisms tested included fo ur erythromycin-resistant isolates of B. pertussis from a single patient, a second erythromycin-resistant strain of B. pertussis from an unrelated pat ient in another state, and 47 nasopharyngeal surveillance isolates of B. pe rtussis from children in the western United States. The results of agar dil ution testing using direct inoculation of the organisms suspended in Muelle r-Hinton broth were within +/-1 dilution of those obtained after overnight passage of the inoculum in Stainer-Scholte medium, which is the traditional method of testing B. pertussis. The Etest method produced MICs similar to those of the agar dilution reference method for three of the four antimicro bial agents tested; the trimethoprim-sulfamethoxazole results were lower wi th Etest, particularly when the direct suspension method was used, Most of the Etest MICs, except for that of erythromycin, were on scale. Disk diffus ion testing using RL-C medium was helpful in identifying the erythromycin-r esistant strains, which produced no zone of inhibition around the disk; sus ceptible isolates produced zones of at least 42 mm, Thus, the antimicrobial susceptibility testing of B, pertussis can be simplified by using the Etes t or disk diffusion on RL-C to screen for erythromycin-resistant isolates o f B. pertussis.