Chapter 111 



MORPHOLOGY AND LIFE CYCLE 



Lack of complete understanding of the distinct morphological char- 

 acteristics of the actinomycetes and of their mode of reproduction has 

 been one of the major causes of the existing confusion concerning the 

 nature and systematic position of this group of microorganisms. The 

 fact that some species of actinomycetes resemble the true fungi in many 

 respects whereas other species resemble the true bacteria more closely, 

 and the fact that actinomycetes are characterized by marked variation in 

 morphology and in cultural characteristics, especially yvhen grown on 

 artificial media, have also contributed to the confusion. 



One of the early students of the actinomycetes, F. Cohn, recorded in 

 1875 that the "ray fungi," a common designation given to this group of 

 organisms, are fungus-like in nature. This point of view was held by 

 a number of subsequent investigators, notably Thaxter in 1891, 

 Lachner-Sandoval in 1898, Berestnew in 1899, Neukirch in 1902, 

 and more recendy Drechsler, Orskov, Jensen, and others. The pro- 

 duction of a very fine mycelium consisting of unicellular branching 

 hyphae definitely emphasized their similarity to the true fungi. 



On the other hand, the unicellular nature of the mycelium, its very 

 fine structure, the resemblance in dimensions of the hyphae and of the 

 spores to those of the bacteria, and the appearance of stained prepara- 

 tions prepared in accordance with bacteriological practice— all tend to 

 suggest that one is dealing here either with bacteria or with bacteria-like 

 organisms. Since most of the early and even the more recent investiga- 

 tors cultivated the actinomycetes on complex organic media, the differ- 

 ences in morphological structures and cultural characteristics tended to 

 be obscured. Marked differences became apparent only with the intro- 

 duction of synthetic media for the growth of actinomycetes and with 

 the development of suitable microscopic techniques for examination of 

 these organisms in an undisturbed state. It has become recognized 

 that the actinomycetes possess morphological properties which not only 

 are distinct from those of the bacteria but which are, within certain 

 limits, fairly constant. 



Staining of Actinomycetes:— In addition to the direct mcdiods of 

 examining the structure of the actinomycete colonies and their growth 

 characteristics, that is, the general methods developed by students of 



