SUMMARY: The high-power microscopical examination of stained specimens of portions of agar plate cultures of bacteria allows bacterial cells and colonies to be studied in their original positions. The disposition and morphology of bacteria from the edges of clear zones on penicillin assay plates under conditions sufficiently standardized permitted a distinction between sensitive and resistant strains of . With a sensitive strain ‘ghost zones’ were formed of partially lysed, distorted and abnormally staining cells; with a resistant strain a ‘seatter zone’ was formed of apparently normal cells in isolated colonies:

For plate assay work, a sensitive strain may be used when only low concentrations of the drug are available, a resistant strain may be expected to give a sharper zone edge, but will require higher concentrations to provide an adequate response.


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