Influences of Media and Inocula on the In Vitro Susceptibility ofHaemophilus influenzaeto Co-Trimoxazole, Ampicillin, Penicillin, and Chloramphenicol

Abstract

The effects of inocula and media on the activities of ampicillin, penicillin, chloramphenicol and co-trimoxazole againstHaemophilus influenzaewere examined in vitro. Two inocula and four media were tested by the disk diffusion, broth dilution, and agar dilution methods. Chloramphenicol activity versusH. influenzaewas least affected by changes in inocula and media, whereas co-trimoxazole was most susceptible to these effects. Filde's and Levinthal's agar dilution tests were most satisfactory for ampicillin. Penicillin was less active on Levinthal's than on Filde's agar. Both ampicillin and penicillin were less active when tested against the higher inoculum. Co-trimoxazole was most active (<1%H. influenzaewas resistant) when tested at an inoculum of 106colony-forming units/ml on diagnostic susceptibility test agar with 5% lysed horse blood added. The majority ofH. influenzaeappeared resistant to co-trimoxazole with increases in the test inocula and/or when tested on brain heart infusion with Filde's, Levinthal's or “low-thymidine” Mueller-Hinton medium.