78 ANTIBIOTIC ACTION OF ANTAGONISTS 



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 

 determined. 



Various modifications of this method have been developed. In one 

 such modification (607), the bacterial cells are suspended for 15 to 30 

 minutes in various dilutions of the active substance j the cells are then 

 centrifuged oflF, washed, and cultured. This procedure can be utilized 

 for substances that have a rapid bactericidal action. Its value is limited, 

 however, by the fact that most antibiotic substances do not kill bacteria 

 so rapidly as do chemical antiseptics (p. 189). 



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 (550). The growth of Ceratosto- 

 mella ulmi was inhibited by several substances, comprising actinomycin, 

 clavacin, and hemipyocyanin in concentration of i: 100,000 (803). 



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. Some of the results obtained are presented 

 in subsequent chapters. 



ISOLATION AND UTILIZATION OF 

 ANTIBIOTIC SUBSTANCES 



The isolation of antagonistic microorganisms from natural sub- 

 strates, the determination of the nature of the antibiotic substances pro- 



