[Evaluation of the sensitivity of Haemophilus to antibiotics by the modified API-ATB system]

A simple micromethod in a liquid medium using the API-ATB system was developed for testing the susceptibility of Haemophilus to antibiotics. To evaluate this method, 50 strains, including 12 beta-lactamase producers, were studied. Results were compared to those obtained using MIC determination in a liquid medium (reference) and an agar diffusion method (routine). For all three techniques, a Mueller-Hinton medium enriched in hemoglobin and NAD was used, and cultures were incubated at 37 degrees C for 24 hours in normal atmosphere. Influence of the inoculum on results was evaluated using the API-ATB method for all antibiotics and MIC determination for ampicillin; the optimal inoculum was found to be 8.10(5) CFU/ml. Beta-lactamase was looked for using the chromogen cephalosporin test associated with the API-ATB system. Values of MICs for the various antibiotics were consistent with previous reports. Paired comparison of techniques showed a 5.3% disagreement rate between API-ATB and MIC, with only 0.5% major discrepancies; in contrast, the disagreement rate exceeded 10% when disk diffusion was compared with the two other techniques. We conclude to the reliability and reproducibility of the API-ATB method which seems capable of improving current routine evaluations of the susceptibility of Haemophilus to antibiotics.