THE NATURE OF A MIXED MICROBIAL POPULATION 41 



toward the bacteria and use the latter as a source of nitrogen j the yeasts 

 secrete a bacteriolytic substance that is also active outside their cells. 

 Various bacteria are able to kill yeasts when they are inoculated into 

 suspensions of the latter in distilled water. The destruction of the 

 fungus Ofhiobolus, the causative agent of the take-all disease of cereals, 

 by soil organisms was believed (312) to be a result of the need of a 

 source of nitrogen by the latter. 



The term "autoantibiosis" has been used (670) to designate the 

 phenomenon of self-inhibition or "staling" of medium as a result of the 

 previous growth of the organism in this medium. 



THE NATURE OF A MIXED MICROBIAL 

 POPULATION 



A mixed microbial population is made up of a great variety of bac- 

 teria, and often also of fungi, actinomycetes, and protozoa j to these are 

 added, under certain conditions, various algae, diatoms, nematodes and 

 other worms, and insects. The specific nature and relative abundance of 

 the various microorganisms making up a complex population in either a 

 natural or an artificial environment depend upon a number of factors, 

 which can be briefly summarized as follows: 



The physical nature of the medium in which the population lives: soil, 

 compost, or manure pile; river, lake, or ocean; sewage system; or 

 peat bog. 



The nature, concentration, and availability of the chemical constituents 

 of the medium used by the microbes as nutrients, especially the ma- 

 terials used as sources of energy and for the building of cell sub- 

 stance. Various organic and inorganic substances, whether complex 

 or simple in chemical composition, favor the development of specific 

 groups of microorganisms capable of utilizing them. For example, 

 sulfur favors the development of specific sulfur bacteria, and cellu- 

 lose favors such organisms as are capable of attacking this complex 

 carbohydrate as a source of energy. In many instances there is con- 

 siderable competition for the available food material. Organisms that 

 possess a greater capacity of attacking the particular compound, or 

 are capable of preventing the development of other organisms by the 

 formation of substances injurious to the latter, usually become pre- 



