An Improved Single-disk Method for Testing the Antibiotic Susceptibility of Rapidly-growing Pathogens

Abstract

The Kirby-Bauer method of disk antibiotic-susceptibility tests is well standardized and accurate, but the method for standardizing the inoculum is rather time-consuming and is often impractical for routine use. This report describes a somewhat more practical, efficient method for inoculating the test plates. A calibrated inoculum obtained from a small volume of broth culture is spread over the agar surface with a seeded agar overlay. Comparison of this new method with the standardized Kirby-Bauer method shows that the zones of inhibition are nearly identical in size, and thus the same zone standards for interpretation can be applied to both methods. Because the inhibitory zone edges were better defined on plates inoculated with the agar-overlay method, there was a slightly better correlation between zone diameters and minimal inhibitory concentrations, especially with bacteriostatic drugs. Tests with swarming Proteus mirabilis were much more satisfactory with the agar-overlay method. The new technic is recommended as a routine method for testing the common Enterobacteriaceae, P. aeruginosa and S. aureus.