80 



ANTIBIOTIC ACTION OF ANTAGONISTS 



Figure 9. Relation between penicillin concentration and inhibition of 

 Stafhylococcus aureus. The penicillin preparation contained 42 Oxford units 

 per milligram, and the incubation period was sixteen hours at 37° C. From 

 Foster (280). 



pared with the control, or it can be measured by a convenient turbi- 

 dimeter. The results obtained by this method are more nearly quantita- 

 tive than those obtained by other methods, as shown in Figure 9. By 

 proper modifications, the length of time required to obtain a satisfac- 

 tory reading can be reduced to four hours (482, 610), or even to 90 

 minutes (280, 554). 



The turbidimetric method has also found application in the stand- 

 ardization of streptomycin. For this purpose, certain noncapsulated 

 cultures of K. fneumoniae are used. Nutrient broth is inoculated from 

 a slant and incubated at 37° C. for 22 to 24 hours. A series of dilutions 

 of streptomycin in nutrient broth is prepared and one-milliliter portions 

 are added in duplicate to tubes containing 9 cc. of broth inoculated with 



2 per cent of the culture. The tubes are incubated at 37° C. for 3 hours, 



3 drops of formalin are added to stop growth, and turbidity is deter- 

 mined in a photoelectric colorimeter. The turbidity measurements are 



