Filter Paper Disc Modification of the Oxford Cup Penicillin Determination. ∗

At the present time biological assay methods are the sole means of determining penicillin potency. A number of such methods are in use in various laboratories; 1 however, the Oxford cup method, 2 with several modifications, 3 has continued to be regarded as a procedure in which a commensurate accuracy and speed may be obtained with a minimum of labor. The following filter paper disc modification should reduce the labor and time involved in setting up this test, while the case with which extra replicates may be run should increase the test's accuracy.

The principal modification lies in the use of a thick filter paper disc saturated with the penicillin sample, substituted for the sample-containing small cylinder used in the Oxford cup method. These discs may be conveniently set up on the seeded plates at a rate of about 6 per minute, a rate considerably more rapid than that in the original method. Another advantage of the discs is that the test plates may be manipulated freely to facilitate reading. Experience with a large number of tests has indicated that the zones of inhibition obtained with the discs are more consistent and more sensitive to variations in the penicillin content of the sample than the zones obtained with the cup method. This improvement may be due to a more consistent contact of the penicillin solution with the agar and to a more even diffusion from the disc.

The test organism, Staphylococcus aureus H† (Oxford strain), tends to produce a somewhat granular growth in nutrient broth. With the use of peptone broth as the seeding medium, a more diffuse growth is obtained and this in turn effects a more even seeding of the test plates.