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
Staphylococcus aureus. With a sensitive strain ‘ghost zones ’ were formed of partially
lysed, distorted and abnormally staining cells ; with a resistant strain a ‘ scatter 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