METHODS OF MEASURING ANTIBIOTIC ACTIVITY 



69 



Figure 

 (962). 



Production of gliotoxin by Trichoderma. From Weindlinc 



by another. The most important methods at present in use are briefly 

 summarized in the following pages. 



The Agar Plate-Dilution Method 



If an unknown antibiotic substance is tested, it is essential to employ 

 several test organisms in order to throw light upon the selective activity 

 of the substance on different bacteria. Nutrient agar media have usually 

 been employed. Sterility is not absolutely essential for this method, al- 

 though it is desirable. The unknown substance is diluted to various con- 

 centrations (i, 0.3, 0.1, etc.j or I, 0.5, 0.25, etc.); these dilutions are 

 added and thoroughly mixed with definite volumes (lo ml.) of sterile 

 agar medium, melted and cooled to 42° to 45° C. The agar is allowed 

 to solidify, and is streaked with three or four test bacteria, among the 

 most common of which are E. coli, E. ty-phosa, Br. abortus, B. subtilis, 

 S. aureus, M. lysodeikticus, and S. lutea. The age of the cultures ( 1 6 to 

 24 hours) is important. The plates are incubated at 28° or 37° C. for 



