GENERAL MORPHOLOGY AND PHYSIOLOGY 59 



to each other and to their host are, in many instances at least, not well under- 

 stood. For example, it is not clear what biological relationship the dif- 

 ferent species of bacteria in a mixed infection bear to each other. In the 

 case of the root nodule organisms of the Leguminosae it is known that there 

 is a mutually beneficial (mutualistic symbiosis, mutualism) relationship 

 between microbe and host but it is not obligatively so, since the symbionts 

 can exist independently of each other. In most diseases due to microbic 

 invasion there is one species of bacterium which acts as the primary cause. 

 It is known that tuberculosis, especially the pneumonic form, usually 

 shows a mixed infection, and it is probable that the associated organisms 

 as bacteria and higher fungi act as predisposing causes, preparing the 

 tissues so as to yield more readily to the invasion of the primary cause, 

 the Bacillus tuberculosis. Such an association may be designated com- 

 pound symbiosis, in which the relationship of the invading organisms 

 (secondary and primary) is mutualistic and the relationship of these to the 

 host is antagonistic. It is known that certain microbic diseases predispose 

 to other microbic invasions, thus we may say that these organisms are 

 mutuaUstically disposed toward each other. 



Since it is possible to cultivate most disease germs in and upon artificial 

 culture media (hence dead organic substances) it is evident that they are 

 only facultatively parasitic. 



In many instances the biological association of bacteria and higher 

 plants and animals is loosely mutualistic, as the bacteria upon roots and 

 rootlets of all plants and the bacteria lining the intestinal tract of animals. 

 The hay baciUus (Bacillus subtilis) is a constant associate with the Gra- 

 mineae and serves an important function, assimilating or binding for the use 

 of the host plant, the free nitrogen of the air. Certain soil organisms 

 (Bacillus megatherium, B. ellenbachiensis, B. mesentericus, B. pyocyaneus, 

 B. prodigiosus, the Azotobacter group, Clostridium pastorianum, certain 

 moulds as Aspergillus niger and Penicillium glaucum) are capable of assimi- 

 lating the free nitrogen of the air thus enriching the soil for the benefit 

 of higher plants. 



