Chapter XII —189— Summary 



pounds as sources of energy, whereas the actinomycetes prefer to utiHze 

 for this purpose the organic nitrogenous compounds. The glucose may 

 thus serve as a buffer, since the acid produced from it tends to neutralize 

 the excessive amounts of ammonia which are produced by actinomycetes 

 and which would soon bring growth to a standstill because of a rapid 

 change in reaction of the medium to highly alkaline, pH 8.6-9.0. 



The metabolic changes produced in the medium by actinomycetes 

 are greatly influenced by the nitrogen source. In general, however, 

 actinomycetes are similar to bacteria and to fungi in their nutrition, in 

 their energy utilization, in the transformation of nitrogenous com- 

 pounds, in the liberation of ammonia, and in cell synthesis. 



Actinomycetes thus form a major group of microorganisms, and com- 

 parable to the other two major groups, the bacteria and the fungi, their 

 activities can be summarized under the following five headings: 



Role in natural processes. 



Causative agents of disease. 



Agents of spoilage and deterioration. 



Utilization for production of enzymes and vitamins. 



Production of antibiotics. 



Role of Actinomycetes in Natural Processes:— The general occur- 

 rence of actinomycetes in all soils and their omnivorous nature suggest 

 their probable importance in soil processes. The facts that they make 

 up as many as 15 to 40 per cent of all colonies developing on the plate, 

 that they occur in the soil at great depth, and that they are favored by 

 arid soil conditions and by an alkaline reaction suggest that, under cer- 

 tain conditions, actinomycetes are concerned in a number of important 

 processes. 



The following soil reactions may be due, to a considerable extent, to 

 the activities of actinomycetes: I. Decomposition of complex plant and 

 animal residues in soils and in composts. 2. Liberation of ammonia 

 from complex proteins. 3. Humification processes accompanied by the 

 formation of black coloring substances, the decomposition of humus com- 

 pounds, and the synthesis of cell material, which further contributes to 

 the formation of soil organic matter or soil humus. 4. Reduction of 

 nitrate to nitrite, but not to atmospheric nitrogen. 5. Favorable effects 

 upon plant growth. For example, when a soil was enriched with actino- 

 mycetes, plant roots were longer. This effect was explained by the 

 greater decomposition of organic soil constituents. The effect was 

 greatest on legume bacteria, which suggested possible assistance to 

 legume bacteria in infecting the plants and causing greater nodule devel- 

 opment (125). 



Actinomycetes appear to be important geological agents, although 

 thei^ role in this respect has not been fully established. Nadson (311), 

 who isolated several actinomycetes from lake muds, found that they 



