SUMMARY: The diffusion of streptomycin in agar gel seeded with staphylococci as in assay methods, results in a clearly defined zone of inhibition. The time required for this definition to develop is about 2 ½ hr. at 37°, but varies greatly with temperature and is equal to the lag period plus approximately 4 times the generation time. After 4–5 generations the young colonies become much less susceptible and grow unimpeded by the rapidly increasing concentration of the antibiotic reaching them. Incubation at temperatures below 37°, although without significant effect on the minimum inhibitory concentration, results in larger inhibition zones because of increased generation times.

In assays and sensitivity tests by diffusion methods, errors will occur if organisms under comparison do not multiply at the same rate.


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