SPECIES CONCEPT IN RKLATION TO ACTINoMYCKTKS 



employed for their cultivation. With t he in- 

 troduction of synthetic media, it became 

 definitely established that the aerial myce- 

 lium-producing actinomycetes comprise a 

 large number of tonus, differing greatly in 

 their physiological and biochemical proper- 

 ties, and to a lesser degree in their morphol- 

 ogy. It was also recognized that, if a suffi- 

 ciently large number of cultures was isolated 

 and examined, many differences would he 

 noted suggesting variability of the type spe- 

 cies. The concept "species-croups," with one 

 culture as the type species, was suggested. 

 Waksman ( 1919) emphasized, therefore, ihat 

 in spite of variation of individual biochemi- 

 cal characteristics of the actinomycetes 

 there are certain well defined properties, no- 

 tably morphology, color of aerial mycelium, 

 and formation of soluble pigments, that char- 

 acterize these organisms, especially when 

 grown on standard synthetic media and un- 

 der carefully controlled conditions of tem- 

 perature and aeration. 



It is easy to pick out a few cultures of 

 actinomycetes (or streptomycetes) which 

 possess characteristic properties that can be 

 recognized as distinct species, and to discard 

 all the others. This was actually done by 

 Waksman and Curtis in their early (1915- 

 1916) classification of actinomycetes, since 

 they were faced with such a large number of 

 freshly isolated cultures that it was impos- 

 sible to consider more than a very small 

 number of them. How many others have 

 acted likewise it is dillicult to say. Should 

 the various intermediate strains be consid- 

 ered, one might be inclined to regard each 

 as a different species, distinct from the 

 others in at leasl one variable property, be 

 it morphological, cultural, or biochemical. 

 With the examination, in recent years, of 

 many thousands of cultures of actinomy- 

 cetes for their antibiotic properties, such an 

 attitude was frequently reduced to an ab- 

 surdity. There are those who contend thai 

 the insistence on permanent characteristics. 



preferably a group of | hem, in describing new- 

 species, would limit greatly our recognition 

 of the growing economic importance of these 

 organisms. Then there are those who reason 

 thai not enough species of actinomycetes 

 have so far been described, thus justifying 

 random descripl ions of many freshly Isolated 

 si rains as new species. 



Even synthetic media did not yield the 

 final answer to the species problem of this 

 group of organisms. Their cultural proper- 

 ties, or growth characteristics in media of 

 different chemical composition, properties 

 that were at first greatly emphasized, were 

 found to be extremely variable. Type cultures 

 were shown to change their specific charac- 

 teristics when grown in artificial media. 

 Saltations and mutations came to play a 

 highly important part in changing such prop- 

 erties. When morphology was recognized at 

 all, it was limited largely to observations on 

 the curvature of the sporophores or to the 

 size and shape of the spores. Drechsler ( I !> 1 !) I 

 was the first to make a detailed study of the 

 morphology of the actinomycetes thai pro- 

 duce aerial mycelium. Unfortunately, he 

 limited his study to a small number of cul- 

 tures; this prevented him from establishing 

 the existence of many specific types which 

 could have been recognized on the basis not 

 only of cultural but also of morphological 

 properties. 



It must be regarded as a considerable step 

 backward when Lieske (1921) completely 

 disregarded the work of Krahisky (1914), 

 Waksman and Curtis (1916), Conn (1917), 

 and Waksman (1919). He believed ihat the 

 classification of actinomycetes was impos- 

 sible, since the properties observed were 

 highly variable. His skeptical attitude to- 

 ward the question of speciation of actino- 

 mycetes was due largely to his use of complex 

 media for t he growth of these organisms, and 

 to a lack of sufficient appreciation of the 

 significance of simple media tor their char- 

 acterizat ion. 



