Influences of media and inocula on the in vitro susceptibility of Haemophilus influenzae to co-trimoxazole, ampicillin, penicillin, and chloramphenicol.

The effects of inocula and media on the activities of ampicillin, penicillin, chloramphenicol and co-trimoxazole against Haemophilus influenzae were examined in vitro. Two inocula and four media were tested by the disk diffusion, broth dilution, and agar dilution methods. Chloramphenicol activity versus H. influenzae was 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. influenzae was resistant) when tested at an inoculum of 10(6) colony-forming units/ml on diagnostic susceptibility test agar with 5% lysed horse blood added. The majority of H. influenzae appeared 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.