Evaluation of Two Standardized Disk Methods for Testing Antimicrobial Susceptibility of Pseudomonas aeruginosa and of the Enterobacteriaceae

Two standardized disk diffusion susceptibility testing methods were compared using Pseudomonas aeruginosa and members of the Enterobacteriaceae. The standard Bauer-Kirby method and the agar overlay method of Barry et al. were compared using carbenicillin (50- and 100-mug disks), gentamicin, polymyxin B, and colistin sulfate. Both methods gave nearly comparable zone sizes with the enteric bacilli. However, with P. aeruginosa the agar overlay method gave zones 1 to 2 mm smaller than the Bauer-Kirby method with all drugs except gentamicin. In spite of these small differences, further examination of minimal inhibitory concentration-zone size correlations indicated that the same interpretive zone standards can be applied to either diffusion technique. For testing carbenicillin against Pseudomonas sp., 50-mug disks were unsatisfactory, especially with the agar overlay method; 100-mug disks were far superior with both methods.