METHODS OF MEASURING ANTIBIOTIC ACTIVITY 67 



the growth of some bacteria in very low concentrations, whereas 

 much larger amounts are required to affect other bacteria and some 

 organisms may not be inhibited at all by the particular substance even 

 in very high concentrations. 



Conditions for the bacteriostatic activity of different antibiotic substances 

 vary greatly. Some substances are not active at all, or their activity 

 is greatly reduced in some media because of the neutralizing effect of 

 certain constituents of the media, such as peptone or /"-amino-benzoic 

 acid. Other agents require the presence in the medium of specific 

 constituents for their activity to become effective. The activity of 

 some is reduced at an acid reaction (287, 1002), whereas that of 

 others is not affected. 



The mechanism of the action of different antibiotic agents is different. 

 Some agents interfere with bacterial cell division, others with bac- 

 terial respiration, and still others with utilization by the bacteria of 

 essential metabolites. 



Many antagonistic organisms produce more than one antibiotic substance. 

 Ps. aeruginosa produces pyocyanase and pyocyanin ; B. brevis, grami- 

 cidin and tyrocidine; Streftomyces antibioticus, actinomycin A and 

 B ; P. notatuniy penicillin and notatin ; Asfergillus fumigatus, spinu- 

 losin, fumigatin, fumigacin, and gliotoxin; Asfergillus flavus, asper- 

 gillic acid and flavicin. The culture filtrate of an antagonistic or- 

 ganism often differs, therefore, in its activity from that of the 

 isolated active substance. 



The course of production of antibiotic substances by two typical antago- 

 nistic organisms is illustrated in Figures 7 and 8. 



In view of the bacteriostatic nature of antibiotic substances, few of 

 the methods commonly used for testing the efficiency of antiseptics and 

 germicides can be employed. This is particularly true of the "phenol 

 coefficient test," which measures the germicidal action of phenol upon 

 E. tyfhosa. The limitations of this method, based on the bactericidal ac- 

 tion of a single substance on a single organism, even as applied to chemi- 

 cal antiseptics have long been recognized (735, 773). 



A number of methods have been developed for determining the ac- 

 tivity of antibiotic substances. They vary greatly, each having its limita- 

 tions and advantages. Because of lack of uniformity in the methods, the 

 results obtained by one are not always comparable with those obtained 



