A model of expert system using Prolog language was developed, to verify the coherence of the results of the antibiotic sensitivity test. Biological knowledge was formalized in three different ways: a credibility coefficient based on epidemiological data is assigned to known observed resistance; co-existent resistances are described with lists of "implicit" resistances, reflecting phenotypes commonly observed within some antibiotic groups; every single or "implicit" resistance is connected to a "gregarius" status, expressing the plasmidic nature of resistance. Applied to Staphylococcus aureus, the expert system is able to detect the inconsistencies of the antibiotic sensitivity test and to identify required knowledges. It therefore allows phenotypic interpretation of results.