New method for antibiotic susceptibility testing.

The most widely used method for routine antibiotic susceptibility testing of clinical isolates of bacteria is the paper-disc method. This has the advantage of simplicity, but to obtain reliable results considerable care is required both in the standardization of the procedure and in the interpretation of the zones of inhibition. A new susceptibility test method is described in this report which retains the simplicity of the paper-disc method but which enables organisms to be tested directly for susceptibility to known concentrations of antibiotic in agar. Organisms may be tested against a single concentration of antibiotic or, alternatively, a range of concentrations may be used to determine the minimal inhibitory concentration. The method utilizes stable pre-prepared materials and is not appreciably more time consuming than the conventional disc method.