THE ACTINOMYCETES, Vol. II 



aval ion of the concepts of "nomenclatural 

 taxonomy" from those of "experimental tax- 

 onomy." It is to be remembered that species 

 are, after all, convenient "artificial creations 

 of human imagination" rather than "real 

 biological entities." Gilmour further sug- 

 gested that "nomenclatural categories of 

 genus, species, variety, etc." are excellently 

 suited for the purpose of "a broad map of 

 the diversity of living things." It would, 

 therefore, be "a great advantage if they were 

 not subject to continued attempts to bring 

 them up to date and to redefine them in 

 evolutionary terms." 



Speciation of Actinomycetes Other than 

 Streptomycetes 



Krassilnikov (1938) wrote, "In spite of the 

 most extensive literature, we have no definite 

 idea concerning the natural systematics of 

 the actinomycetes, nor a single opinion of 

 their structure and development." The re- 

 cently accumulated information leads us to 

 conclude, however, that we need not be so 

 pessimistic. 



According to Pridham (1959), there are 

 now known more than 100 genera of actino- 

 mycetes and well over 1500 subgeneric names 

 and specific, or subspecific, epithets. Some of 

 the descriptions of these forms are good, 

 others lack essential details, and many are 

 worthless. Morphological criteria are be- 

 lieved to play an important role in separa- 

 tion at the generic level (Fig. 1), with a 

 gradual intergradation in complexity of re- 

 productive units. The actinomycetes are 

 looked upon as a heterogeneous group of 

 organisms, ranging from the simple myeo- 

 cocci and the seemingly more complex no- 

 cardiae to the straight or flexuous strepto- 

 mycetes and the verticillate forms, and from 

 the relatively simple micromonosporae to 

 forms such as Waksmania, Actinoplanes, and 

 Streptosporangium (the latter two genera pos- 

 sibly having some affinities with the chy- 

 trids). Some of these organisms have definite 



affinities with true bacteria, others with both 

 bacteria and microfungi, and still others with 

 phycomycetous fungi. 



This heterogeneity is further emphasized 

 by the facts that the actinomycetes contain 

 forms that are anaerobic, microaerophilic, or 

 aerobic; forms that fragment and those that 

 do not; and forms that produce aerial my- 

 celium and those that do not. Pridham sug- 

 gested that some of the present concepts 

 centered around the three genera Actino- 

 myces, Nocardia, and Streptomyces be ac- 

 cepted. Thus included in the Actinomyces 

 would be the anaerobic to microaerophilic 

 forms; in the Nocardia, the aerobic types 

 that either form no aerial mycelium or pro- 

 duce an aerial mycelium that generally has 

 no catenulate spores; and in the Streptomy- 

 ces, the aerobic forms that generally produce 

 catenulate spores. 



Although time and again taxonomists have 

 emphasized that an effective system of clas- 

 sification should be based upon criteria that 

 are expressed in consistently reproducible re- 

 sults, this has hardly been applied, at least 

 so far as our present knowledge is concerned, 

 to the species characterization of actino- 

 mycetes. Many "new species" have been de- 

 scribed on the basis of a single difference — 

 frequently a quantitative variable — from 

 "old species." One often wonders what the 

 composition of the medium, the conditions of 

 growth, and the natural variability observed 

 so frequently among duplicate cultures have 

 to do with these distinguishing properties. 



The species concept among the actinomy- 

 cetes must be considered as the continuity 

 between different groups of organisms desig- 

 nated as species, with various transitional 

 forms bridging the gaps between species. The 

 concept of natural classification applies to 

 actinomycetes perhaps better than to many 

 other bacterial groups: there are the chemi- 

 cal approach (chemical composition, pres- 

 ence of specific chemical compounds), the 

 morphological approach (type of aerial my- 



