44 



INTERRELATIONSHIPS AMONG MICROORGANISMS 



Destruction by one microorganism of toxic substances produced by an- 

 other, thereby enabling the continued development of various mem- 

 bers of the microbiological population. 



Modification of the physiology of one organism by another. In the presence 

 of certain bacteria, Clostridium granulobacter-fect'movorum forms 

 lactic acid instead of butyl alcohol (845). The presence of Clos- 

 trid'tum acetobutyVicum in cultures of bacteria producing dextro-lactic 

 acid and laevo-lactic acid causes such bacteria to form the inactive lac- 

 tic acid (870) ; intimate contact of the bacteria is essential, the use of 

 membranes preventing this effect. The presence of A. aero genes 

 modifies the optimum temperature for nitrogen-fixation by Axoto- 

 bacter (749). Pigment formation by P^. aeruginosa may be weak- 

 ened when the latter is grown together with other organisms. E. coli 

 may lose the property of fermenting sugars when grown in the 

 presence of paratyphoid organisms (453). 



Some associations of microorganisms are not so simple. The complex 

 system of animal infection by more than one organism, with the result- 

 ing complex reactions in the animal body, is a case in point. 



The effect of one organism upon the activities of another can be illus- 

 trated by the results of the decomposition of complex plant material by 

 pure and mixed cultures of microbes (Table 7). Trichoderma, a fungus 



TABLE 7. DECOMPOSITION OF ALFALFA BY PURE AND MIXED 

 CULTURES OF MICROORGANISMS 



From Waksman and Hutchings (938). 



