[Antibiotic susceptibility testing by the disk method. Comparative evaluation of 5 commercial systems].

The systems studied were: bioMerieux, Difco, Institut Pasteur Production, Oxoid (media, discs, standards of interpretation). Each batch was tested for pH value and calcium and magnesium concentrations. The activity of 19 antibiotics was determined by use of three reference strains (E. coli ATCC 25922, S. aureus ATCC 25923, P. aeruginosa ATCC 27853). The most suitable inoculum was defined after testing three different methods of preparation. The susceptibility of each strain to each antibiotic was studied 30 times using each system. The reproducibility of the diameters with each system and the intersystem variance was calculated. This allowed groups of homogeneity between the different systems to be defined. The results were interpreted according to the various existing standards (manufacturer, French committee of SFM, WHO) yielding an overall discordance of 0,29%, of little practical importance. Finally the reliability of each system was evaluated according to WHO, data. The Difco medium gave the most accurate results with the other media tending to produce excessively large zones of inhibition.