40 INTERRELATIONSHIPS AMONG MICROORGANISMS 



nated as commensalisms. The various stages of transition from obligate 

 parasitism to true saprophytism can be represented as follows: 



Obligate parasitism (cer- Facultative parasitism (spe- Modified parasitism; 



tain bacteria, smut fungi) — > cies of Fusarium, Rhizoc- — > hosts may derive some — > 

 tonia, and Actinomyces^ benefit (certain mycor- 



rhiza) 



Balanced parasitism (vari- True symbiosis (root- True saprophytism (auto- 



ous mycorrhiza) — > nodule bacteria, lichen — » trophic and heterotrophic 



formations) bacteria and fungi). 



The phenomena of antagonism do not fit exactly into the above 

 scheme but are parallel with it: the injurious effects of one organism 

 upon another range from antagonism of varying degrees of intensity 

 to the actual living or preying of one organism upon another. The lat- 

 ter may be classified with the phenomena of parasitism and disease pro- 

 duction. 



Microorganisms inhabiting the soil live in a state of equilibrium 

 (943). Any disturbance of this equilibrium results in a number of 

 changes in the microbial population, both qualitative and quantitative. 

 The ecological nature of this population found under certain specific 

 conditions, as well as the resulting activities, can be understood only 

 when the particular interrelationships among the microorganisms are 

 recognized. Because of its complexity, the soil population cannot be 

 treated as a whole, but some of the processes as well as some of the 

 interrelations of specific groups of organisms can be examined as sepa- 

 rate entities. Some have received particular attention, as the relations 

 between the nonspore-forming bacteria and the spore-formers, the ac- 

 tinomycetes and the bacteria, the bacteria and the fungi, the protozoa 

 and the bacteria, and the relations of the bacteria and the fungi to the 

 insects. 



The term "synergism" has been used to designate the living together 

 of two organisms, resulting in a change that could not be brought about 

 by either organism alone (425). Microbes living in association fre- 

 quently develop characteristics which they do not possess when living 

 in pure culture. Schiller (797, 798), for example, found that when beer 

 yeasts are placed together with tubercle bacteria in a sugar-containing 

 but nitrogen-free medium, the yeasts develop antagonistic properties 



