4 ACTINOMYCETES 



Because of these variations among strains of a single species, it has often 

 been difficult to identify freshly isolated cultures with already established 

 types. This has been particularly difficult when the latter were known by 

 published description only. But even when type cultures were available for 

 comparison, they may have undergone such marked changes as a result of 

 long cultivation in artificial media that they no longer represent the original 

 isolate. This may be illustrated again with S. griseus. When the original 

 strain of this culture, which was isolated in our laboratory in 1915, was 

 tested in 1946 for antibiotic formation, it was found to be unable to produce 

 any antibiotic. When the same culture, which was deposited in the Baarn 

 (Holland) Collection in 1920, was tested, it was found to be able to form 

 an antibiotic that was not streptomycin. When the original culture kept in 

 our collection was irradiated by Kellner, a strain that was capable of pro- 

 ducing streptomycin was obtained. One may, therefore, only speculate 

 whether this proves that the original 1915 strain possessed the capacity of 

 producing streptomycin and lost it, or that it did not possess it at all, but 

 on irradiation produced a mutant that acquired such capacity. 



Need for Standard Media and Standard 

 Conditions of Growth 



The use of standard media, comprising both synthetic and complex or- 

 ganic media, for the growth of actinomycetes is particularly important in 

 any attempt to characterize and identify these organisms. The same is 

 true of the need for a standard environment, notably aeration and tem- 

 perature control. Synthetic media have found extensive application in the 

 study of the morphology and physiology of these organisms. This is true 

 especially for cultural characterization of these organisms, particularly 

 pigmentation. Organic media, often quite specific, are used for obtaining 

 supplementary evidence, especially for strains that do not grow at all or 

 grow only very weakly on the common inorganic media. 



The great majority of actinomycetes are aerobic, very few are anaerobic, 

 many are microaerophilic. To supply proper aeration, the organisms are 

 grown on the surface of solid media, or in shallow liquid media, or in a 

 thoroughly aerated submerged condition. 



Temperatures of 25°-30° are usually used for incubation of the great 

 majority of streptomycetes, nocardias, and micromonospora. Frequently 

 somewhat different temperatures are used for seed production and for anti- 

 biotic formation under factory conditions. Pathogenic organisms require 

 37°C, and thermophiles need 50°-60°C or even higher temperatures. 



Among the stable morphological properties of actinomycetes essential for 

 purposes of characterization and classification, one must list the structure 

 and subsequent changes in the vegetative mycelium, the production and 



