76 ANTIBIOTIC ACTION OF ANTAGONISTS 



measurement of the potency of bactericidal agents. Partial inhibition 

 can be determined by plating for the number of viable bacteria, as com- 

 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 in nature than those obtained by other methods (281, 520), as 

 shown in Figure 9. By proper modifications, the length of time re- 

 quired to obtain a satisfactory reading can be reduced to four hours 

 (4^5) 572)5 or even to 90 minutes (281, 520). 



S fecial Methods 



Certain methods were found to be specific for measuring the action of 

 certain substances. The ability of tyrothricin to hemolyze red blood 

 cells served as the basis for measuring the potency of this substance 

 ( 1 72) : the tyrothricin content is calculated from the amount of hemoly- 

 sis by the unknown and is read from a standard curve. The inhibition 

 of growth of a |3-hemolytic streptococcus, group A, as measured by 

 hemolysin production has been used for assaying the potency of peni- 

 cillin (715, 989). Penicillin can also be estimated by its inhibition of 

 nitrite production by S. aureus cultures (343). The antiluminescent 

 test has been utilized (716, 717) not only for measuring the activity of 

 certain substances but also for determining their possible usefulness. 

 The results of a comparative study of a number of antibiotic substances 

 by this and the dilution methods are brought out in Table 10. 



Various other methods have been suggested for measuring the activ- 

 ity of antibiotic substances. Some are based upon interference with a 

 given physiological function of the test organism such as dehydrogenase 

 activity and respiration, others upon the prevention of growth of the 

 test organism (pneumococcus) in semi-solid tissue culture medium 

 (387). Although only a single method is usually employed in the con- 

 centration and standardization of a given antibiotic substance such as 

 penicillin, it is often advisable to check the results by another method, 

 especially where several test organisms are used, in order to ascertain 

 that one is still dealing with the same type of chemical compound. 



Some of the above methods can also be adapted to the determination 

 of the concentration of antibiotic agents in the body fluids and exudates 

 (719). 



