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THE ACTINOMYCETES, Vol. I 



tial cells" are artifacts; she believed that 

 aerial hyphae may arise by budding from 

 any substrate (vegetative) hyphae. Wilkin 

 and Rhodes observed that the nature of the 

 medium influences the morphological forms 

 produced by a Streptomyces: on synthetic 

 media, the cycle proposed by Klieneberger- 

 Nobel was confirmed ; on complex media, the 

 formation of "initial cells" and "secondary 

 mycelium" was suppressed, however. The 

 "primary mycelium" pi'oliferates on com- 

 plex media to give "chlamydospores," which, 

 on germination, give rise to typical "pri- 

 mary mycelium." Further information on 

 the life cycles of actinomycetes is given by 

 Wilkin and Rhodes, and Roach and Silvey. 



Dickenson and Macdonald made electron 

 microscopic observations on submerged 

 cultures of two species of Streptomyces. The 

 evidence obtained tended to confirm the 

 theory of Klieneberger-Nobel, that at an 

 early stage in the life history of the organ- 

 ism concerned, fusion occurs between por- 

 tions of the same or different hyphae, cul- 

 minating in the formation of "initial cells." 



Further studies on problems of recombi- 

 nation of nuclear material and on the life 

 cycles of actinomycetes in general, are dis- 

 cussed in Chapter G. 



Colony Formation 



The grovvth of an actinomycete on a solid 

 or in a liquid medium results in the forma- 

 tion of a mass of unicellular mycelium us- 

 ually designated as a "colony." This is not 

 a colony in a true sense, since it is not an 

 accumulation of many cells, but rather a 

 mass of branching hlaments which originated 

 from a spore or from a bit of mycelium. The 

 two types of mycelium making up a colony 

 of a streptomyces often show fundamental 

 differences in appearance, composition, and 

 biological activities. The substrate or \'eg;r- 

 tativc mycelium grows into the medium, 

 whereas the aerial mycelium grows on the 

 surface; the well-(le\'eIopc(l spomliiting hy- 



phae and the reproductive spores are pro- 

 duced in the aerial mycelium. Some actino- 

 mycetes form only the substrate mycelium, 

 whereas others produce both types. Some 

 aerial mycelium-forming cultures may lose 

 this property, and may thus be distinguished 

 from nocardias only by certain physiological 

 properties, as pointed out elsewhere. Some 

 nocardias, on the other hand, also produce a 

 typical aerial mycelium, as shown by Gordon 

 and Mihm (1958). 



According to Henrici (1930), surface colo- 

 nies produced by various actinomycetes are 

 of two general types: (a) One type is char- 

 acteristic of those strains that form a highly 

 developed, extensi\'ely branching mycelium, 

 notably members of the genus Streptomyces. 

 Colonies of this type are very firm, almost 

 cartilaginous in consistency, and adhere to 

 the solid substrate, because the mycelium 

 grows into that substrate; when touched 

 with a wire loop, the colony does not break 

 but separates from the substrate as a unit. 

 In cross section, such colonies usually have 

 a slightly conical form and show marked 

 radial foldings. At first their surface may be 

 glossy or matted, but, if aerial spores are 

 de\'eloped, the surface becomes covered with 

 chalk-like powder which, as the colony grows 

 older, may acquire various shades of color. 

 The powdery spores frequently appear in 

 concentric rings, (b) The second type of 

 colony is characteristic of the strains that do 

 not form an extensive mycelium, notably 

 members of the genus Nocardia. Their thal- 

 lus has a tendency to break up into hyphae 

 of variable length, and in certain strains most 

 of the growth may consist of short filaments, 

 resembling in appearance pleomorphic bac- 

 terial strains. Colonies of this type are less 

 tenacious than those of the first; they often 

 have a mealy consistency and tend to crum- 

 ble wluni touched with a wire loop. 



Actinomycete colonies are usually round 

 and smooth, or much folded and lichenoid in 

 appeai'ance. When examined under the mi- 



