98 



A MANUAL OF THE PENICILLIA 



facturers. Strain Q-176, or modifications of it which have been subse- 

 quently produced in individual laboratories, is still generally used for the 

 commercial production of penicillin. 



Fig. 31. Cu]) met Ik id of assaying penicillin titres. The cups are placed on the 

 surface of an agar plate heavily and uniformly seeded with Staphylococcus, they are 

 then filled with samples to be tested, and the plates are incubated for 16 hours at 37°C. 

 The diameters of the resulting zones of inhibition are a function of the amount of 

 penicillin present. (After Schmidt and Moyer, 1944.) 



Microbiological Assay 



The method of assaying penicillin first developed by Heatley in 1941, 

 has been generally followed (fig. 31) with certain modifications. Improve- 

 ments upon the original methods have been reported by Foster (1943a and 

 1943b), Schmidt and Moyer (1944), Schmidt (1945-1946), Hunter and 

 Randall (1944), and others. Outstanding among such improvements has 

 been the adoption of filter paper discs in place of the porcelain, glass, or 

 metal cylinders formerly employed (Vincent and Vincent, 1944 and Loo 



