Summary: A microphotometric technique is described by which the penicillinase content of a single bacterium in a stable microdrop is measured. The microdrop contains a buffer, benzylpenicillin and bromocresol purple. As the penicillinase hydrolyses the penicillin to penicilloic acid, the concentration of the cationic (purple) form of bromocresol purple decreases. This decrease is measured by a specially designed single-beam microphotometer, and, from the change in light transmission and the volume of the drop, the total penicillinase content of the drop is calculated. The micro-photometer and the calibration of the assay with soluble penicillinase preparations are described.


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