72 



THE ACTINOMYCETES, Vol. I 



filled with protoplasm; toward the center of the 

 thallus, the vacuoles increase in size, and may be 

 associated with the presence of metachromatic 

 granules. The vegetative hj^phae of the mycelium 

 are far larger than those of ordinary bacteria of 

 the acid-fast group; the hyphae also lack the uni- 

 formit}' in diameter generally characteristic of the 

 true bacteria. 



2. The aerial mycelium produced on suitable 

 media usually occurs as a mat of discrete fructi- 

 fications. Each of these represents a well -char- 

 acterized sporogenous apparatus, consisting of a 

 sterile axial filament bearing branches in an open 

 racemose, or dense capitate, arrangement. The 

 primary branches may function directly as sporo- 

 genous hyphae, or they may proliferate branches 

 of the second and of higher order; sporogenesis, in 

 the latter case, is confined to the terminal ele- 

 ments, the hyphal portions below points of attach- 

 ment of branches remaining sterile. 



3. Two tendencies in the development of fructi- 

 fications were recognized: (a) one leading to an 

 erect dendroidal type, in which successively pro- 

 liferated fertile elements undergo processes of 

 sporogenesis in continuous sequence; (b) the other 

 leading to a prostrate, racemose type, in which 

 sporogenesis is delayed in the older branches until 

 the younger branches have also attained their 

 final extension. The majority of species show these 

 tendencies combined in different ways. 



4. The sporogenous hyphae of many species 

 are coiled in peculiar spirals. These exhibit pro- 

 nounced specific characteristics in the number, 

 diameter, and obliquity of their turns, and es- 

 pecially in the direction of rotation, which may be 

 dextrorse or sinistrorse. This phenomenon was 

 later found not to be constant, however, but to 

 vary with the composition of the medium. 



5. Sporogenesis begins at the tips of the fertile 

 branches and proceeds basipetally. In some species 

 the process involves the insertion of septa, which 

 are, in certain cases, relatively very massive, and 

 in others, so thin as to be barely discernible. 



0. Ciranules which possess the staining prop- 

 erties and uniformity of size characteristic of 

 nuclei are readily differentiated in the spores of 

 many sj)ecies; they generally occur singlj', but in 

 the larger spores of a few forms, two are often 

 found occupying diagonally opposite positions. 

 As in the vegetative thallus, metachromatic 

 granules occur in the aerial mycelium, being verj^ 

 rarely found in spores or sporogeneous hyphae but 

 becoming very abundant in degenerate sterile 



7. Peculiar spherical structures appear regu- 



larly in some forms, both in the sterile axial 

 hyphae, where they may contain either a medium 

 septum or a number of peripheral metachromatic 

 granules, and in the sporogenous hyphae, where 

 they are associated with the regularly spaced 

 septa. 



8. The spores germinate readily in proper 

 media, producing from one to four germ tubes, 

 the approximate number being more or less char- 

 acteristic of the species. 



0rskov divided the actinomycetes into 

 three morphological groups: 



I. Those that produce an undivided substrate 

 mycelium and an aerial mycelium which breaks 

 up into bodies that possess the quality of spores. 



II. Those that produce an initially undivided 

 substrate mycelium. After having reached a cer- 

 tain point, it divides by septa into rod-shaped 

 elements; these continue to multiply with a char- 

 acteristic "angular" growth; aerial mycelium may 

 or may not be formed; in the former case its ele- 

 ments resemble those of the vegetative mycelium. 



III. Those that produce a substrate mycelium 

 resembling that of group I, but devoid of aerial 

 mycelium and producing spores borne singly at 

 the distal end of short mycelial branches. 



According to 0rskov the angular growth 

 into which the members of group II pass 

 after the formation of an initial mycelium is 

 similar to the process of cell division in the 

 mycobacteria and corynebacteria. This simi- 

 larity makes it impossible to draw a sharp 

 line of demarcation between true bacteria 

 and actinomycetes. 



Jensen emphasized that the nocardias, or 

 proactinomycetes, as he designated them, 

 represent a heterogeneous collection of types, 

 standing between the mycobacteria and 

 corynebacteria, on the one hand, and the 

 streptomyces, on the other. 



The morphological structure of actino- 

 mycetes depends largely upon (a) the nature 

 of the organism, (b) the composition of the 

 medium, (c) conditions of growth, especially 

 aeration, and (d) presence of growth-stimu- 

 lating and growth-inhibiting factors. The 

 thallus consists of a homogeneous cytoplasm 

 which, as it grows older, becomes vacuolated 



