Disc diffusion test for the determination of semi-quantitative substrate profiles of beta-lactamases

The incorporation of beta-lactamases from crude, cell-free lysates incorporated into agar plates used for antibiotic sensitivity testing with sensitivity discs results in a characteristic decrease in the inhibition zones. The degree of zone reduction for a particular antibiotic depends on the substrate specificity of the enzyme involved and the concentration of the enzyme in the agar. By using this technique a distinction can be made between cephalosporinases, penicillinases and broad-spectrum enzymes. In a number of cases, the identification of the enzyme subclass is possible, provided that the relevant penicillin and cephalosporin discs are included in the test. For the elucidation of substrate profiles for routine and epidemiological purposes this method can replace other, more elaborate techniques, such as the iodometric, spectrophotometric and acidimetric methods. An analysis by this method is presented for 126, unrelated, consecutive clinical isolates.