DEHYDRATED CULTURE MEDIA 205 



tures of Micrococcus pyogenes var. aureus (PCI 1203) in the turbidimetric assay 

 of bacitracin. 



The discovery of penicillin^ and of streptomycin^ and the widespread use of 

 these and other antibiotics in the treatment of many bacterial infections neces- 

 sitated the development of methods for their detection and assay. A number of 

 methods for the microbiological assay of antibiotic drugs have been described. 

 These include serial dilution methods with liquid or solid media, turbidimetric 

 methods and the Oxford cup or cylinder plate method. 



The cylinder plate method for the assay of penicillin was first described by 

 Abraham et al.* and later modified by Foster and Woodruff^ and by Schmidt 

 and Moyer.^ This method depends upon the diffusion of the antibiotic material 

 from steel or porcelain cups placed upon agar plates which have been seeded with 

 the test organism. Inhibition of growth of the organism occurs in the proximity 

 of the cup, and the diameter of the inhibited zone varies with the concentration 

 of the antibiotic in the material being tested. This method is commonly employed 

 for assaying commercial penicillin preparations and is also adapted for the detec- 

 tion of penicillin in body fluids of patients under penicillin treatment. 



The cylinder plate method has given uniform results which may be duplicated. 

 Continued study of this method of penicillin assay showed that a modification of 

 the Schmidt and Moyer formula^ resulted in more accurate readings; also, that 

 if dextrose is present in the seed layer but omitted from the base layer the zones 

 of inhibition are more clear-cut and defined. These modifications were adopted 

 by the Food and Drug Administration^ as standard for penicillin assay using 

 Micrococcus pyogenes var. aureus as the test organism. 



Plates are prepared the same day samples are to be tested, using the cylinder 

 method of assay for antibiotics by adding 21 ml. of sterile Bacto-Penassay Base 

 Agar to sterile petri dishes (100 x 20 mm.). After the base layer has solidified 

 it is overlaid with 4 ml. of Bacto-Penassay Seed Agar previously inoculated with 

 a carefully standardized dilution of the test organism. 



To rehydrate the medium, suspend 25.5 grams Bacto-Penassay Base Agar in 

 1000 ml. of cold distilled water and heat to boiling to dissolve the medium com- 

 pletely. Distribute in 500 ml. quantities in one-liter Erlenmeyer flasks and steri- 

 lize in the autoclave for 15 minutes at 15 pounds pressure (121°C.). The final 

 reaction of the medium will be pH 6.6. 



One pound of Bacto-Penassay Base Agar will make 17.7 liters of medium. 



1 The Compilation of Tests and Methods of ^ PfQc. Soc. Exp. Biol. Med., 55:66:1944. 

 Assay for Antibiotic Drugs, Federal Security * Lancet, 2:177:1941. 



Agency, Food and Drug Administration. ^ j_ Bact., 46:187:1943. 



2 Brit. J. Exp. Path., 10:226:1929. a j. Bact., 47:199:1944. 



BACTO 



PENASSAY SEED AGAR (B263) 



DEHYDRATED 



Bacto-Beef Extract 1.5 g. 



Bacto- Yeast Extract 3 g. 



Bacto-Casitone 4 g. 



Bacto-Peptone 6 g. 



Bacto-Dextrose 1 g. 



Bacto-Agar 15 g. 



Bacto-Penassay Seed Agar conforms to the medium specified by "The Com- 

 pilation of Tests and Methods of Assay for Antibiotic Drugs,"i Federal Security 

 Agency, Food and Drug Administration, for antibiotic assay. It is recommended 



