92 



THE ACTIXOMYCETES, Vol. I 



Streptomyces 



This genus is characterized by the per- 

 manently undivided character of its sub- 

 strate myceUum and by the formation of 

 spores in its aerial mycelium. Instances do 

 occur, however, in which the ability to form 

 aerial mycelium is lost by mutation (Erik- 

 son, 1948), and also in which organisms with 

 the typical spore-apparatus of streptomyces 

 produce a soft-textured, easily fragmenting 

 substrate mycelium, and even turbidity in 

 liquid media (Jensen, 1931; 0rskov, 1938). If 

 such strains should permanently lose their 

 ability to form aerial mycelium, they would 

 obviously become indistinguishable from 

 true Nocardia. Wright (1937) described the 

 formation of corynebacterium-like \'ariants 

 in streptomyces. Jones (1949) also described 

 such strains, of which the classification seems 

 arbitrary. 



Erikson considered the streptomyces to be 

 among the most successful of all microorgan- 

 isms. To have attained such uni\'ersal dis- 

 tribution, it is clear that the organism itself 

 (the genotype) must possess a balance of 

 favorable characters. She described the most 

 important of these as follows: 1. The re- 

 productive rate and efficiency of its mecha- 

 nism. 2. The duration of life of the organism. 

 3. The resistance of the organism to exterior 

 influences. The most valuable property is 

 that of producing large numbers of special, 

 resistant cells (spores). This is the great 

 advantage possessed by the genus Strepto- 

 myces over the genus Nocardia. 



The substrate mycelium of streptomyces 

 develops homogeneously, giving rise to a 

 tough-textured, cartilaginous growth, with a 

 smooth or rough and lichenoid surface; it 

 tends to adhere strongly to the medium. The 

 substi'ate mycelium does not divide during 

 the course of de\'elopment, and gi\'es rise to a 

 somewhat thicker aerial mycelium. The lat- 

 ter is formed (>xp(H'ially on synthetic media. 

 The aerial hyphae produce straight or cur\'ed 

 sporulating branches, giving rise to conidia. 



by the process of "fragmentation." The 

 process of "segmentation," or oidia forma- 

 tion, may also occur in the vegetative myce- 

 lium (Jensen, 1931; von Plotho, 1940). The 

 substrate mycelium may produce chlamy- 

 dospores. 



According to Erikson (1953), the forma- 

 tion by streptomyces of a thick, glossy, 

 tough growth, completely devoid of aerial 

 mycelium, that adheres so closely to media 

 represents a common instance of physiologi- 

 cal adaptation. This vegetative growth may 

 occur on many substrates rich in nitrogen 

 and in phosphorus. These variations may be 

 temporary, the characteristic pattern of 

 growth being restored by returning the 

 organism to a suitable, better balanced, and 

 simpler medium; sterile soil is quite suitable. 



Erikson (1948, 1953) noted further that 

 the absence of readily fermentable carbohy- 

 drates in the medium resulted in a marked 

 lowering of the variability rate when cul- 

 tures of streptomyces were plated out on 

 similar media containing glucose, sucrose, 

 starch, lactate, or acetate. The toxic condi- 

 tions arising in unaerated or solid cultures 

 were said to be a result of the accumulation 

 of organic acids following the dissimilation 

 of sugars: "The secondary colonies that 

 arise at the periphery of established growths 

 on old plates frequently exhibit a distorted 

 and shrunken appearance, with straight 

 aerial filaments instead of spirals, colorless 

 or only faintly pigmented, and with unusual 

 modes of vegetative branching. Such cases 

 of unstable reversible modifications com- 

 monly make up the greater part of the het- 

 erogeneity of streptomycetes capable of 

 synthesizing complicated structures from 

 minimal media." 



Drechsler considered the actinomycete 

 mycelium to be definitely septated, the 

 hyphae l)eing diA'ided into short sections. 

 This ])h(Mi()m(Mion is jjarticularly striking in 

 cullur(>s b(>longing to the nocardias, ])ut 

 appears only seldom among the strepto- 



