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Interpretive reading of antibiotic disc agar diffusion tests indicates the resistance mechanisms, if any, expressed by a bacterium. An expert system for determining resistance mechanisms using rapid automated antibiotic susceptibility tests has been developed. The β-lactan susceptibility of each of 300 strains of clinically significant species of enterobacteria, displaying natural and acquired resistance mechanisms, was determined by disc agar diffusion and by a rapid automated method of susceptibility testing associated with an expert system. For every strain, the conclusion of the expert analysis of the automated test was compared with the commonly accepted interpretation of disc agar diffusion tests. Of the 300 strains studied, 275 were similarly interpreted (91.7% agreement). The susceptible and naturally β-lactan-resistant phenotypes (wild phenotypes) were equally recognized by both methods. Similarly, the results of the two methods concurred for most of the acquired resistance phenotypes. However, for 25 strains (8.3%) the results diverged. The expert system proposed an erroneous phenotype (5 strains), several phenotypes including the correct one (17 strains), or no phenotype (1 strain). For 2 strains the natural resistance mechanism was not detected at first by the automated method but was subsequently deduced by the expert analysis according to bacterial identification. These results demonstrate that satisfactory interpretive reading of automated antibiotic susceptibility tests is possible in 4 to 5 hours but requires careful selection of the antibiotics tested as phenotypic markers.