Sl'KCIKS CoNCKPT IN RELATION TO A.CTINOMYCETES 



Krassilnikov (1949) insisted that the shape of 

 the spore, as seen in the lighl microscope, 

 should be recognized as the major criterion 

 for species differentiation. It is doubtful, 

 however, whether Krassilnikov's various 

 "longisporus" and "globisporus" types, with 

 their many subtypes, can greatly facilitate 

 the solution of the problem of species char- 

 acterization. The cultural properties of these 

 organisms still offer si >f the most im- 

 portant criteria for species differentiation. 

 There is also now available sufficienl addi- 

 tional information concerning morphology, 

 such as formation and branching of the 

 sporophores, formation and nature of spores, 

 and especially the spore surface as shown by 

 the electron microscope, to make possible 

 the use of these criteria not only for supple- 

 mentary but often for major characteriza- 

 tion of the species. 



Several factors have thus contributed to 

 the confusion in establishing and recognizing 

 species of act iiiomycetes : (a) lack of clearly 

 defined morphological characters; (b) great 

 variability of these organisms; (c I occurrence 

 of numerous transition types; (d) ease of 

 formation of mutants; (e) lack of sufficiently 

 recognizable type species; (f) lack of empha- 

 sis upon species-groups and upon type cul- 

 tures; and (g) insufficient recognition of the 

 formation of well defined chemical com- 

 pounds which could be used as additional 

 criteria for species characterizat ion. 



The suggestion that closely related spe- 

 cies be placed in "species-groups" or "aggre- 

 gate-species" has recently been gaining con- 

 siderable attention. Such a unit should be 

 characterized by various reproducible prop- 

 erties under standard c litions of culture. 



Baldacci et al. I L953, L956) suggested that 

 micromorphological criteria, namely, seg- 

 mentation and branching of vegetative my- 

 celium, presence or absence of -pores, and 

 arrangement of sporophores, be used for 

 generic classification. The genus Streptomy- 

 ces was then divided, on the basis of pigmen- 



tat ion of i he vegetat ive and aerial mycelium, 

 into a number of "-eric-." each of which 

 was further subdivided into specie-. Gause 

 d <il. (1957) made use of the "series" con- 

 cept and created a number of groups based 

 on the pigmentation of the aerial mycelium. 



When so many different cultures of ac- 

 t iiiomycetes can be isolated easily from natu- 

 ral substrates, it is but natural thai various 

 intermediate type- should be found and that 

 established species should tend to overlap 

 one another. \i one were to isolate only a 

 small number of cultures, it would be simple 

 to recognize a few well defined species. But 

 when hundreds of similar strains are found 

 in nature and when many of them show only 

 minor variations from one another, varia- 

 tions which are not important enough to 

 warrant creation of new species hut are 

 nevertheless variations from the established 

 type, the difficulties mount rapidly (Fig. 2). 



When study is based upon a single strain, 

 a particular species may he described as hav- 

 ing a yellow or yellowish aerial mycelium. 

 Another strain may produce, on the same 

 medium, an aerial mycelium only a shade 

 different in color from the original type; 

 this pigment may he designated as sulfur- 

 yellow, cream-yellow, saffron-yellow, or even 

 brownish, all other physiological and mor- 

 phological properties being similar. Would 

 one he just ilied in calling such a new strain a 

 different species? The answer is definitely 

 "no." One culture may produce a strong 

 tyrosinase react ion, and another only a weak 

 reaction, as indicated by pigmentation with 

 potato, gelatin, and other protein media. 

 ( me would he inclined to accept these as 

 mere quantitative variations a 1 Iowa hie for an 

 established species. This must he recognized, 

 since it is well known that had the test been 

 repeated in another laboratory, where the 

 medium mighl he slightly different in coin- 

 position, the method of -1 erili/ai a f the 



medium different, or the age and origin of 

 the inoculum different, these variations 



