An economic approach to the MIC.
Résumé
The determination of the Inhibitory Concentration in Diffusion (ICD) is proposed as an alternative to the agar dilution Minimal Inhibitory Concentration (MIC) that is time-consuming and cumbersome for routine use. Based on the technique of the disk diffusion test, it consists in calculating a continuous variable, the ICD, corresponding to the antibiotic concentration in the agar at the edge of the inhibition zone. Six antibiotics were tested (ampicillin, cefotaxime, erythromycin, gentamicin, nalidixic acid and rifampicin) each against 17 to 51 strains of enterobacteria, Staphylococcus aureus and Enterococcus spp. and six other antibiotics (cefsulodin, ceftazidime, imipenem, piperacillin, ticarcillin and tobramycin), against 13 to 25 strains of Pseudomonas aeruginosa. A total of 284 antibiotic-strain combinations were tested. Three different antibiotic charges were obtained for each antibiotic by cutting commercial disks in two and four equal pieces. The ICD was calculated for each strain from the size of inhibition zones around a full disk, a half and a quarter of a disk. Concurrently, the MIC was performed, using a conventional agar dilution method. There was a good correlation between the two methods and reproducibility for the ICD proved to be correct. This reliable technique is very efficient both in terms of laboratory time and cost of materials and could be proposed for widespread use in clinical laboratories.