66 ANTIBIOTIC ACTION OF ANTAGONISTS 



on agar slants in order to obtain abundant spore material for the inocu- 

 lation of stationary cultures. For submerged cultures, special spore sus- 

 pensions are produced by growing the organisms in shaken cultures. 



The cultures must be tested carefully in order to establish the opti- 

 mum activity when the culture filtrate is cooled and extraction of active 

 substance is started. 



METHODS OF MEASURING THE ACTIVITY OF 

 ANTIBIOTIC SUBSTANCES 



It has long been recognized that the evaluation of bacteriostatic and 

 bactericidal substances is controlled to a considerable extent by the 

 methods employed. These methods are based upon the following fac- 

 tors: (a) proper selection of the test organism, (b) composition of the 

 medium used for testing activity, (c) time of action, (d) conditions of 

 carrying out the test, and (e) nature of the active substance. The results 

 obtained in a comparison of substances containing the same active prin- 

 ciple may not be very reliable when different agents are compared, since 

 these vary greatly in their specific action upon different bacteria. This is 

 especially true of antibiotic agents. 



In most of the work on chemical disinfectants, which are primarily 

 bactericidal agents, the death rate of the viable cells has been used as a 

 basis for evaluation. Different substances have been compared with a 

 standard, ordinarily phenol. Since antibiotic and chemotherapeutic 

 substances are primarily bacteriostatic in action, the inhibition of the 

 growth and multiplication of the test organism is commonly used as a 

 basis for their evaluation. 



In any attempt to select a single standard method for measuring 

 quantitatively the activity or potency of an antibiotic substance, it is es- 

 sential to recognize several pertinent facts, which may be briefly sum- 

 marized as follows : 



Antibiotic (antibacterial, antimicrobial) substances are primarily bac- 

 teriostatic (or fungistatic) in their action; they are bactericidal (or 

 fungicidal) only to a limited extent, although some substances may 

 possess marked bactericidal properties. 



Antibiotic substances are selective in their action; they are able to inhibit 



