42 INTERRELATIONSHIPS AMONG MICROORGANISMS 



ing bacteria and a source of nitrogen for many fungi. The ability of 

 bacteria to concentrate in solution those nutrients that are present 

 only in mere traces enables animal forms (protozoa) to exist at the 

 expense of the bacteria ( io6). 



Influence upon the oxygen concentration available for respiration. This 

 involves the phenomenon first observed by Pasteur (709) of con- 

 sumption of oxygen by aerobic bacteria, thus making conditions fa- 

 vorable for the development of anaerobes. 



Symbiotic interrelationships, where both organisms benefit from the asso- 

 ciation. The three most important examples found in nature are: 

 (a) the phenomenon of symbiosis between root-nodule bacteria and 

 leguminous plants; (b) mycorrhiza formations between certain 

 fungi and higher plants; (c) lichen formation between algae and 

 fungi. Certain other interrelations are not strictly symbiotic, but are 

 found to fall between groups a and c; here belong nitrate reduction 

 accompanied by cellulose decomposition and nitrogen-fixation with 

 cellulose decomposition, carried out in each case by two specific 

 groups of organisms. 



Production by one organism of growth-promoting substances that favor 

 the development of other organisms. The formation of riboflavin by 

 anaerobic bacteria in the digestive tract of herbivorous animals is an 

 interesting and highly important phenomenon in the nutrition of such 

 animals. The production of bacterial growth stimulants by yeasts 

 and the beneficial action of mixed populations upon nitrogen-fixation 

 by Avcotohacter are other illustrations of this general phenomenon. 

 The presence of specific bacteria has been found necessary for the 

 sporulation of certain yeasts and for the formation of perithecia by 

 various Aspergilli (825). Various other processes of association have 

 also been recognized (940). 



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-fectinovorum forms 

 lactic acid instead of butyl alcohol (873). The presence of Clos- 

 tridium acetobutylicum in cultures of bacteria producing dextro-lactic 

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

 tic acid (897) ; intimate contact of the bacteria is essential, the use of 

 membranes preventing this effect. Pigment formation by Ps. 



