METHODS OF MEASURING ANTIBIOTIC ACTIVITY 81 



plotted against the concentrations of streptomycin and compared with 

 the standard. 



S fecial Methods 



Special 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 8 1 ) : 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 (747, 1022). Penicillin can also be estimated by its inhibition of 

 nitrite production by 6". aureus cultures (350). The antiluminescent 

 test has been utilized not only for measuring the activity of certain sub- 

 stances but also for determining their possible usefulness. The results 

 of a comparative study of a number of antibiotic substances by this and 

 the dilution method are brought out in Table 9. 



Other methods suggested for measuring the activity of antibiotic 

 substances are based upon interference with a given physiological func- 

 tion of the test organism such as dehydrogenase activity and respiration 

 (562) upon the prevention of growth of the test organism (pneumo- 

 coccus) in semisolid tissue culture medium (392), or upon the diffu- 

 sion of the antibiotic placed upon the surface of inoculated semisolid in 

 tubes and measurement of the depth of clear zone (27). Although only 

 a single method is usually employed in the concentration and standard- 

 ization of a given antibiotic such as penicillin or streptomycin, it is often 

 advisable to check the results by another method, especially where sev- 

 eral test organisms are used, in order to obtain an antibacterial spectrum 

 that will demonstrate that one is still dealing with the same type of 

 chemical compound. 



The results obtained by the various methods for determining the 

 relative potency of different antibiotics lend themselves readily to 

 statistical analysis (68, 513, 514). 

 ' Methods have also been developed for detection of chemotherapeu- 



