METHODS OF MEASURING ANTIBIOTIC ACTIVITY 69 



K0HPO4 S-65 gm. 



MgS04.7HoO 0.98 gm. 



ZnS04.7H20 o.oi 1 5 gm. 



FeS04.7HoO o.oiii gm. 



CUSO4.5H.O 0.0064 gm. 



MnClo^HoO 0.0079 gm. 



Distilled water to make lOOO ml. 



fU 6.95 



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 antibiotics. 



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; some substances are also 

 markedly bactericidal (or fungicidal). 



