Diffusion of teicoplanin and vancomycin in agar

Teicoplanin, although more active than vancomycin [by minimum inhibitory concentration (MIC)], produces smaller inhibition zones in sensitivity testing with 30-μg disks. Our data support the hypothesis that this is due to lower diffusion of teicoplanin in agar media. After 6 hr of incubation, ≈70% of vancomycin, but only 20% of teicoplanin entered the agar from a paper disk charged with 30 μg of antibiotic. This is due to a difference between the diffusion coefficients: 0.47 mm²/hr for teicoplanin and 0.72 mm²/hr for vancomycin. With the methodology used in this work, it is possible to calculate the range of concentrations of the antibiotic occurring at times likely to include the critical time-the time when the inhibition zone is formed-of most strains at any given distance from the reservoir. One can thus estimate the breakpoint diameter for a given MIC breakpoint; for example, an MIC breakpoint of ≤4 μg/ml would correspond to a ≥15-mm breakpoint diameter for vancomycin (30-μg disk) and a ≥13-mm breakpoint diameter for teicoplanin (30-μg disk).