40 INTERRELATIONSHIPS AMONG MICROORGANISMS 



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 utiHzing 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 for 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- 

 dominant. Proteins, starches, and sugars can be acted upon by a 

 great variety of microorganisms. The predominance of one group 

 may depend not only upon the chance presence of the particular or- 

 ganism or its capacity for more rapid growth, but also upon its ability 

 to form alcohols, acids, and other products that influence the growth 

 of other organisms. 



Environmental conditions favorable or unfavorable to the development 

 of specific organisms. Of particular importance in this connection 

 are temperature (thermophilic vs. mesophih'c organisms), oxygen 

 supply (aerobic vs. anaerobic organisms), moisture content (bac- 

 teria and fungi vs. actinomycetes), reaction (acid-sensitive vs. acid- 



