Antimicrobial Susceptibility of Hemophilus Influenzae

Seventy strains of Hemophilus influenzae (49 capsulated and 21 noncapsulated) were tested for in vitro susceptibility to eight antibiotics and two sulfa drugs. Inoculum size has a marked influence on the apparent susceptibility to penicillin, ampicillin, sulfadiazine, and sulfisoxazole. If the Harper and Cawston effect is not taken into account in sulfa testing, hemophilus will appear falsely resistant. Plate dilution testing with a large inoculum results in apparent resistance of hemophilus to ampicillin, whereas all strains are highly susceptible to ampicillin by tube dilution testing. With optimal methodology, Hemophilus influenzae appears susceptible to levels of ampicillin, tetracycline, chloramphenicol, erythromycin, sulfadiazine, and sulfisoxazole which can be achieved with usual therapeutic doses. Lincomycin is not effective, and most strains are resistant to cephalexin, a cephalosporanic acid derivative for oral administration. Levels of 10 to 20 µg per milliliter kanamycin are required for inhibition. Testing conditions have a marked influence on the apparent susceptibility or resistance of Hemophilus influenzae to antimicrobial drugs. Because routine laboratories do not employ the necessary conditions for obtaining meaningful susceptibility testing of this fastidious organism, the physician should not be misled by possibly erroneous antibiotic sensitivity reports.