Waksman — 154 — Actinomycetes 



of plant and animal residues in composts of stable manures or plant 

 materials, the temperature rises rapidly, and the various organic constitu- 

 ents, notably the carbohydrates, decompose rapidly. The numbers of 

 actinomycetes increase rapidly with a rise in the temperature of the 

 compost. It is to be expected, therefore, that these organisms would play 

 an important part in the processes of decomposition taking place in these 

 composts. That this is so, is brought out in Table 41. The degree of 

 decomposition can be measured either by a reduction in dry matter or 

 by the increase in the ash content of the compost, since the ash accumu- 

 lates with the decomposition of the organic constituents. The greatest 

 decomposition was brought about by the complex natural population 

 of the manure. This was followed by decomposition by pure cultures 

 of fungi. The actinomycetes stood in their abilities to decompose the 

 compost materials midway between the fungus culture and the bacteria. 



Of even greater interest is the effect of these organisms on the lignin 

 in the compost. Lignins are highly resistant to microbial decomposi- 

 tion. Together with the proteins they contribute to the formation of 

 black humus in soils and in composts. As has been pointed out, actino- 

 mycetes apparently have the capacity to attack these resistant complexes. 

 In the case of the fungi and the bacteria there was an increase in the 

 concentration of the lignin parallel to the increase in ash content and 

 to the decrease in total dry material. The actinomycetes, however, 

 brought about a much smaller increase in lignin content, pointing again 

 to the destruction of this material by the actinomycetes. 



Conn emphasized the importance of actinomycetes in the decompo- 

 sition of organic residues in the soil. His conclusion was based upon 

 observations that the actinomyces colonies developing on an agar or 

 gelatin plate made up 20 per cent of the total number of colonies 

 from cultivated soils, as compared to 37.5 per cent of the colonies from 

 sod soils. The longer the period of time during which grass was 

 grown on a soil, the larger was the percentage of actinomyces colonies. 

 In an experiment on the effect of grass roots on the relative abundance 

 of actinomycetes as compared to the total number of microorganisms 

 developing on the plate, it was found that the actinomyces content of 

 an untreated soil remained almost constant throughout the experiment, 

 namely, about 2,900,000 colonies per gram of soil. In the soil receiv- 

 ing grass roots, the numbers increased to 6,000,000 in 2 weeks and 

 remained nearly at that height for 10 months, or throughout the ex- 

 periment. The stimulating effect upon the development of actinomy- 

 cetes was caused both by dead grass roots mixed with the soil as well 

 as by grass growing in the soil. 



