366 ACTINOMYCETES 



tomyces are erroneously placed in many of our lists of pathogenic 

 actinomycetes. 



Streptomyces species, mostly of soil origin, are the basis of most 

 of the monographic work on the actinomycetes, e.g., Drechsler," 

 Lieske,^* Waksman,-^ 0rskov.^^ This genus is made up of strictly 

 aerobic forms, which fragment little or not at all, which reproduce 

 largely by conidia in chains, and which are not acidfast. The conidia 

 are not formed on all media by all strains. Especially do they fail 

 to form conidia on media rich in nutrients, but in poorer media, such 

 as Czapek's, conidia usually form quickly with most species. These 

 are the most mold-like actinomycetes and it is understandable that 

 most mycologists, knowing only these organisms, see little reason to 

 consider actinomycetes as bacteria. 



As was stated, species of Streptomyces form an important part of 

 the soil flora. They are more numerous in soils containing an abun- 

 dance of organic matter than in poorer soils. The reaction of the 

 soil seems to be the principal limiting factor, growth being inhibited 

 by even slight amounts of acidity. They are, therefore, practically 

 absent in sour waterlogged soils, as in acid bogs and high-moor peat. 

 They are actually most numerous near the surface, but may extend 

 to a greater depth than the bacteria, so that deeper in the soil they 

 form a larger proportion of the total microbic population. 



They do not fix atmospheric nitrogen, nor do they convert ammonia 

 to nitrates. On the contrary, they reduce nitrates to nitrites, in which 

 form nitrogen is said to be assimilated by them. But the reduction 

 is not carried on to the formation of free nitrogen. They break down 

 proteins to simpler compounds, so that directly or indirectly they are 

 active in ammonification. 



The soil Streptomyces are perhaps more important in their activity 

 in decomposing the more complex carbohydrates, since practically 

 all species break down starch, and many of them chitin and cellulose. 

 Thus they are well adapted to initiate the decomposition of dead 

 plant or animal matter, since they can attack the most complex and 

 abundant of the organic compounds present: protein, starch, cellu- 

 lose, and to some extent lignin. Chitin is also decomposed by soil 

 Streptomyces, and the very large number of these chitinovorous 

 actinomycetes is indicated -^ by the fact that several colonies of 

 chitin-destroying actinomycetes develop on plates poured from soil 

 dilutions of 1:1,000,000. The very slow and imperfect decomposition 

 of plant matter in acid peat bogs may be due in part to the absence 

 of large numbers of actinomycetes. 



