Evaluation of the E test for antimicrobial susceptibility testing of Actinobacillus actinomycetemcomitans

The E test was compared with an agar dilution method for antimicrobial susceptibility testing of Actinobacillus actinomycetemcomitans on Mueller-Hinton Haemophilus test medium. Eighty recent isolates of A. actinomycetemcomitans from 40 subjects and 3 reference strains were studied. Excellent agreement was obtained between the E test and the agar dilution test for benzylpenicillin (98.8%), ampicillin (97.5%), cefaclor (100%), cefuroxime (100%), tetracycline hydrochloride (100%) and ciprofloxacin (98.7%); 76.0% agreement was obtained for metronidazole and only 5.2% for cotrimoxazole. This study indicates that the E test is a reliable method to determine minimal inhibitory concentrations of antimicrobials against A. actinomycetemcomitans, except for metronidazole and cotrimoxazole.