Waksman — 188 — Actinomycetes 



to 65 °C., especially in composts, whereas others, such as those found in 

 abundance in muds and in lake and river bottoms, thrive at rather low 

 temperatures. Under unfavorable conditions, actinomycetes grow only 

 slowly and poorly; this has often raised the question concerning their 

 active participation in a given process. Most of them are sensitive to 

 an acid reaction (pH 6.0) and are favored by an alkaline reaction (pH 

 7.0 to 7.5) of the medium. 



Actinomycetes produce a variety of pigments. The black pigment 

 formed on protein media may function as an oxidizing agent, and on 

 this basis, the suggestion has been made that actinomycetes play an im- 

 portant role in the formation of humus in the soil. They occur in soil 

 at considerable depths, where they may exceed in numbers the other 

 groups of microorganisms. These facts have led to the suggestion that 

 actinomycetes play an important role in soil processes. 



Of the four genera now recognized among the actinomycetes— 

 Actinomyces, Nocardia, Streptomyces, and Micromonos'pora— the animal 

 pathogens are found largely in the first, the anaerobic genus, and to 

 some extent in the second. The plant pathogens are found in the third. 

 The water forms and the high-temperature compost forms are found 

 largely in the fourth. The last three genera occur in great abundance 

 in soils, where they make up nearly 25 per cent of the total population 

 of microorganisms developing on the ordinary agar plate. They occur 

 in the dust and on the surface of grasses and foodstuffs. Their relative 

 abundance in close proximity to the roots of plants is due not so much 

 to their particular preference for living roots as to the fact that they find 

 nourishment in the dead residues and excreta of the roots. 



With the rapid progress, within recent years, of our knowledge of 

 antibiotic substances, and with the recognition that actinomycetes may 

 play an important role in the production of such agents, new interest 

 was aroused in the nutrition of these organisms. The introduction of 

 the submerged culture method for their cultivation has made possible 

 not only their rapid and abundant growth but also the study of many 

 physiological reactions not previously recognized. 



In order to obtain abundant growth, sufficient energy material must 

 be applied by proteins, carbohydrates, or organic acids; proper sources of 

 nitrogen, either organic or inorganic; and certain minerals, notably potas- 

 sium, magnesium, phosphorus, sulfur, and iron, are also necessary. 

 Certain forms are capable of producing vitamin-like substances favoring 

 the growth of other microorganisms. Many are able to produce anti- 

 biotic substances injurious to the growth of other organisms. 



Under comparable conditions of nutrition, actinomycetes may pro- 

 duce as much growth and decompose as much of the substrate as do 

 some of the common fungi and bacteria. Carbohydrates, such as glu- 

 cose, favor the growth of the organism and the utilization of proteins 

 and amino acids. The efi^ect, however, is different from that upon 

 fungi, since the latter prefer the carbohydrates to the nitrogen com- 



