54 ISOLATION AND CULTIVATION OF ANTAGONISTS 



pathogenic organisms. Although it had been previously established that 

 many spore-forming bacteria are capable of producing substances that 

 have antibacterial properties, as shown by the work of Pringsheim 

 (738), Much (664), and others, Dubos was the first to succeed in iso- 

 lating in crystalline form the active substances involved and in demon- 

 strating their chemical nature. He utilized for the isolation of the or- 

 ganisms the soil enrichment culture method. This consisted in adding 

 repeatedly various pathogenic bacteria to a soil in which, as a result, 

 antagonistic organisms developed that were capable of destroying the 

 bacteria i these organisms were then isolated by appropriate procedures. 

 These investigations, as well as the work of Fleming (261 ) later fol- 

 lowed by other British investigators (5) on the antibacterial properties 

 of molds belonging to the Penkillium notatum group, served as the di- 

 rect stimulus to numerous studies. The entire series of studies led to 

 the development of simple methods for the systematic isolation of 

 microorganisms capable of inhibiting the growth of fungi and bacteria, 

 both pathogenic and saprophytic, and for separating many of the anti- 

 biotic substances produced by these organisms. 



METHODS OF ISOLATING ANTAGONISTIC 

 MICROORGANISMS 



Several methods are now available for the isolation of antagonistic 

 microorganisms from natural substrates such as soil, stable manure, 

 composts, sewage, water, and food products. These methods are dif- 

 ferent in nature, but they are all based on the same principle, that of 

 bringing a living culture of a bacterium or fungus into close contact with 

 a mixed natural population, thereby allowing certain members of this 

 population to develop at the expense of the added culture. 



Soil Enrichment Method 



By this method a soil is enriched with known living pathogenic bac- 

 teria. Fresh garden or field soil is placed in glass beakers or pots, and 

 the moisture of the soil is adjusted to optimum for the growth of aerobic 

 bacteria, which is about 6s per cent of the water-holding capacity of the 

 soil (20 to 50 per cent of the moist soil) j the containers are covered 



