72 



ANTIBIOTIC ACTION OF ANTAGONISTS 



Figure 7. Production of gliotoxin by Trlchoderma. From Weindling 

 (989). 



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

 summarized in the following pages (583). 



A gar Streak-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, and thus to determine the anti- 

 biotic spectrum characteristic of each substance. Nutrient agar media 

 have usually been employed. Sterility is not absolutely essential for 

 this method, although it is desirable. The unknown substance is diluted 

 to various concentrations (i, 0.3, o.i, etc.; or i, 0.5, 0.25, etc.) ; these 

 dilutions are added and thoroughly mixed with definite volumes 

 (10 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, B. mycoides, B. 

 suhtilisy S. aureus, M. lysodeikticus, S. lutea, M. fhlei, as well as 

 various other bacteria and fungi. The age of the cultures (16 to 



