AN ECONOMIC-APPROACH TO THE MIC

The determination of the Inhibitory Concentration in Diffusion (ICD) i s proposed as an alternative to the agar dilution Minimal Inhibitory C oncentration (MIG) that is time-consuming and cumbersome for routine u se. Based on the technique of the disk diffusion test, it consists in calculating a continuous variable, the ICD, corresponding to the antib iotic concentration in the agar at the edge of the inhibition zone. Si x antibiotics were tested (ampicillin, cefotaxime, erythromycin, genta micin, nalidixic acid and rifampicin) each against 17 to 51 strains of enterobacteria, Staphylococcus aureus and Enterococcus spp. and six o ther antibiotics (cefsulodin, ceftazidime, imipenem, piperacillin, tic arcillin and tobramycin), against 13 to 25 strains of Pseudomonas aeru ginosa. A total of 284 antibiotic - strain combinations were tested. T hree different antibiotic charges were obtained for each antibiotic by cutting commercial disks in two and four equal pieces. The ICD was ca lculated for each strain from the size of inhibition zones around a fu ll disk, a half and a quarter of a disk. Concurrently, the MIC was per formed, using a conventional agar dilution method. There was a good co rrelation between the two methods and reproducibility for the ICD prov ed to be correct. This reliable technique is very efficient both in te rms of laboratory time and cost of materials and could be proposed for widespread use in clinical laboratories.