METHODS OF TESTING IN VIVO ACTIVITY 83 



action of antibiotic substances. A suspension of washed bacterial cells in 

 saline or other suitable solution, or a 5-to-i2-hour-old broth culture of 

 the test organism, is treated with various dilutions or concentrations of 

 the active substance. After incubation at 37° C. for i to 24 hours, the 

 number of living cells is determined. If the active substance has lytic 

 properties or if the test organism undergoes lysis readily, the readings 

 are simplified. If no lysis occurs, the treated bacterial suspension or cul- 

 ture is streaked or plated out. The streaking procedure gives only a 

 relative idea of the extent of bactericidal action. If 50 to 90 per cent 

 killing of cells Is to be taken as a unit of measurement, the culture Is 

 plated on a suitable medium and the actual number of surviving cells 

 are determined. 



Some of the foregoing methods can also be utilized for measuring 

 the fungistatic and fungicidal properties of antibiotic substances. Pro- 

 tective fungicides may first function as fungistatic agents, others func- 

 tion better either as fungicidal or as fungistatic agents, and still others 

 show either a high or a low for both. The growth of Ceratostomella 

 ulmi was inhibited by actinomycin, clavacin, and hemipyocyanin in con- 

 centration of 1:100,000 (771, 949). 



METHODS OF TESTING THE IN VIVO ACTIVITIES 

 OF ANTIBIOTIC SUBSTANCES 



Ordinary pharmacological, bacteriological, and pathological proce- 

 dures are used for testing the toxicity and activity of antibiotic sub- 

 stances In the animal body. 



In order to determine the amount of an antibiotic required for the 

 treatment of a certain infection, It is essential to know not only the re- 

 sponse of the organism causing the Infection but also the sensitivity 

 of the particular strain Involved. It is also essential to determine the 

 concentration of the antibiotic In the body fluids. A number of methods 

 have been proposed for this purpose, especially for penicillin and strep- 

 tomycin. 



Most of these represent various modifications of the agar diffusion 

 and serial dilution methods, using a hemolytic streptococcus or some 

 other suitable test organism, such as S. aureus or B. subtilis. In some 



