46 INTERRELATIONSHIPS AMONG MICROORGANISMS 



Degree of tolerance of the immune or resistant varieties and of the less re- 

 sistant or more sensitive forms to attack by disease-producing or- 

 ganisms 



Fitness for survival of microbes that are able to adapt to a symbiotic form 

 of life, such as leguminous plants or mycorrhiza-producing plants, 

 and those that are not so adapted 



Survival of parasitic forms that require living hosts for their development, 

 as contrasted with saprophytes that obtain their nutrients from min- 

 eral elements or from dead plant, animal, and microbial residues 



Various special types of competition, for example, competition between 

 strains of root-nodule bacteria (Rhizobium), whereby one strain 

 checks completely the multiplication of other strains, even outside the 

 plant, the dominant strain then becoming responsible for all the 

 nodules produced, as shown by Nicol and Thornton (637). 



These phenomena of competition are found not only in natural sub- 

 strates, such as soil and water, but also in artificial media. When several 

 microbes are growing in the same culture medium, some will be re- 

 pressed in course of time whereas others will survive and take their 

 place. This is due to the fact that these microbes compete for the use of 

 the same nutrients or that conditions, such as reaction, oxygen supply, 

 and temperature, are more favorable to some organisms than to others. 

 Another phenomenon may also be involved, that some organisms may 

 produce toxic substances that repress the growth of others. In artificial 

 media, slowly growing tubercle bacteria, diphtheria organisms, and 

 others will be repressed by the rapidly growing saprophytes. Under 

 aerobic conditions, aerobic bacteria and fungi will repress yeasts and 

 anaerobic bacteria, whereas under anaerobic conditions the reverse will 

 take place. An alkaline reaction will favor the development of bacteria, 

 an acid reaction will favor the growth of fungi. 



ANTAGONISTIC INTERRELATIONSHIPS 



When two or more organisms live together, one may become antag- 

 onistic to the others. The composition of the medium and the conditions 

 of growth influence the nature and the action of the antagonist 5 its 

 metabolism and cell structure may become modified or the cell itself 

 may be destroyed (174). In urine, for example, staphylococci may be- 



