S62 ACTINOMYCETES 



it is also true that the ability to form pigments may be gradually 

 lost or lessened by cultivation, although this ability may in some 

 cases be entirely or partially revived. But a given strain will for 

 months at a time give exactly the same color production on the same 

 medium carefully and accurately made up. Synthetic media are 

 largely used for this. We have here the same problem of variation, 

 better called degeneration, that we have with the dermatophytes. 

 Because of this physiological degeneration it is doubtful that type 

 cultures will be of any more value in identifying these aerobic actino- 

 mycetes physiologically than are type cultures of the dermatophytes 

 morphologically. 



Stanier -'' has made a study of the variations of Streptomyces, in- 

 cluding sudden changes in pigment production. He noted that vari- 

 ants rarely occur in colonies developing from mycelium but fre- 

 quently occur in colonies from conidia. These variations are of the 

 type that occurs in many giant colonies of molds, yeasts, bacteria, 

 and smut sporidia. These variants were manifested by differences 

 in size of colonies, pigmentation (three pigments or lack of them in 

 various combinations in one of the species studied), type of aerial 

 mycelium, conidia or lack of them, tendency to autolysis, and tend- 

 ency to further variation. That the production of variants is any 

 more common, however, in the actinomycetes as a group than in 

 other organisms is not evident to the present author, who has ob- 

 served thousands of colonies of actinomycetes of several weeks' in- 

 cubation and, although variant sectors are sometimes seen, they do 

 not seem to occur more frequently than they do in colonies of molds 

 and bacteria of equal age. 



Most of the actinomycetes grow well on all the usual bacteriological 

 culture media. Some of the pathogenic strains grow very poorly, and 

 some have never been cultivated, but these are exceptions. For the 

 most part they prefer an alkaline medium, and many species are 

 sharply inhibited by relatively slight degrees of acidity. Jensen ^ 

 has described an acidophilous species. For most soil species the 

 limiting pH is 5.0. With most strains on most media there is a 

 tendency for the medium to become more alkaline during their 

 growth. They vary somewhat in their temperature requirements, 

 but most varieties will grow almost equally well at room temperature 

 and in the incubator. A few soil forms are distinctly thermophilic, 

 growing readily at 60° C. Such forms have also been isolated from 

 heating hay. In general, the upper limits are the same as for bac- 

 teria, about 42° C. Most forms are strictly aerobic, but one of the 

 pathogenic forms is anaerobic. This is discussed elsewhere. 



