Interpretative reading: recognizing the unusual and inferring resistance mechanisms from resistance phenotypes

If isolates are speciated and if a sufficient range of antibiotics is teste d, underlying resistance mechanisms can often be inferred from the antibiog ram data. This allows: (i) anomalous combinations of phenotype and organism to be reconsidered; (ii) prediction of further antibiotics that deserve te sting; and (iii) the suppression of susceptibilities that are anomalous in the light of the inferred mechanism. This 'interpretative reading' is widel y undertaken in France but is largely precluded in the UK by limited specia tion and the testing of narrow ranges of antibiotics. Nevertheless, UK labo ratories should be aware of: (i) grossly anomalous combinations of species and phenotype, demanding reference laboratory confirmation; (ii) useful ind icator drugs, where resistance implies a mechanism conferring other resista nces that may be less obvious in direct tests; and (iii) antibiotics that a re prone to select resistant mutants of particular species during therapy. Details of these combinations of organism and resistance are presented. Rel ationships between antibiogram and mechanism are also presented to allow fu ll interpretative reading for those testing wide panels of drugs versus spe ciated isolates.