3-Dimensional susceptibility testing of beta-lactam antibiotics.

A technique for demonstrating the substrate specificities of bacterial beta-lactamases is presented. Enzymatic degradation of beta-lactam antibiotics can be detected by cutting a wall of inoculum into the agar 3 mm from an antibiotic disc. Inactivation of the drug as it diffuses through the wall of inoculum results in distortion of the resulting inhibition zone. This procedure can be applied to the Stokes, Bauer-Kirby, International Collaborative Study (ICS) and Calibrated Dichotomous Sensitivity Test (CDS) methods so that they provide information about any detrimental effect of a pathogen on beta-lactam antibiotics, in addition to the effect the antibiotics have on the pathogen. The additional information generated by this simple procedure should increase the predictive value of in-vitro tests.