METHODS OF ISOLATION 59 



ganisms antagonistic to many bacteria and fungi causing plant and ani- 

 mal diseases have been isolated (644, 646). 



For the isolation of bacteria antagonistic to fungi, the latter are 

 grown on potato agars until they have spread over the plate j particles 

 of moist soil are then placed on the surface of the mycelium, and the 

 plates are incubated in a moist chamber. Bacteria lysogenic to the fungi 

 grow out of the soil and gradually dissolve the mycelium until the en- 

 tire surface of the plate becomes free of the hyphae of the fungus. By 

 transferring some of the material from the lysed spots, pure cultures of 

 bacteria have been obtained that are capable of producing destructive 

 effects upon the fungi, similar to the action of the particles of soil. 



To these four methods may be added the "forced antagonism" 

 method of Schiller (798), previously referred to, which consists in feed- 

 ing a culture of an organism with another one, thereby forcing the sec- 

 ond to develop the capacity of destroying the first. 



Isolation of Antagonistic Microorganisms from Soil 



By means of the foregoing methods, as well as various modifications 

 of them, it was possible to demonstrate that soils, composts, and water 

 basins contain an extensive population of microorganisms possessing 

 antibacterial and antifungal properties. When E. coli was used as the 

 test organism, it was found that although this organism was capable not 

 only of surviving but actually of multiplying in sterile soil, it died off 

 very rapidly when added to fresh soil. The rate of its destruction was 

 greatly increased with every subsequent addition of washed bacterial 

 cells to the soil. This was accompanied by the development of certain 

 antagonistic microbes capable of destroying E. coli in pure culture. 



A large number of fungi, actinomycetes, and bacteria possessing an- 

 tagonistic properties have thus been isolated. The nature of the test or- 

 ganism was found to be of great importance in this connection. When 

 Stafhylococcus aureus y S. lutea, and B. subtilis were used, a large num- 

 ber of antagonists could readily be isolated. With E. coli, however, a 

 much smaller number of microbes thus isolated possessed antagonistic 

 properties. Certain other gram-negative bacteria, like Brucella abortus, 

 were more sensitive than E. coli, whereas certain gram-positive bac- 



