360 ACTINOMYCETES 



forms are not perfectly constant and species may be found which 

 will form one type at one time or on one medium, and the other type 

 at another time or on a different medium. The growth on agar slants 

 in general varies as does the character of the colonies. 



Colonies on agar media that do not promote conidia formation 

 are somewhat difficult to distinguish from bacterial colonies, as are 

 subsurface colonies of most actinomycetes. The radiating mycelium 

 can often only be seen when the colony is examined under the micro- 

 scope. The colonies of non-fragmenting species, however, are al- 

 ways adherent and after a little experience one can usually distin- 

 guish colonies of bacteria from those of actinomycetes without the 

 use of a microscope. 



On liquid media also, two general types of growth may be recog- 

 nized, which again correlate roughly with the types of colonies de- 

 scribed above. The non- fragmenting, tenacious colony types usually 

 grow submerged in the liquid, and form small fluffy masses of my- 

 celium either at the bottom of the tube or adherent to the sides. 

 The fragmenting, mealy colony types tend to form a dry wrinkled 

 pellicle floating on the surface. But again these characters are not 

 altogether constant, and the correlation of these characters is by no 

 means perfect. 



One of the most striking and important of the cultural characters 

 of the actinomycetes is their production of pigments. Three types 

 may be recognized: that which develops in the spores, that which 

 is retained in the mycelium, and that which diffuses into the medium. 

 The spores are usually white or grey, sometimes a distinct brownish- 

 grey or olive-grey. The mycelium may be nearly colorless, having 

 a rather indefinite yellowish translucent appearance, or it may be 

 brilliantly pigmented. Practically all the colors of the rainbow may 

 be found if a number of species are studied; blue, green, yellow, 

 orange, and red. The production of pigments by actinomycetes, 

 especially by the genus Streptomyces, has been studied particularly 

 by Krainsky, by Waksman, by Millard and Burr, and by Conn.^ 

 Pigment production is often used to distinguish species. 



In addition to the pigments of the mycelium, many species form 

 soluble pigments. These are usually of the same color as the pig- 

 ment retained in the mycelium but they may be different. Yellow 

 and orange are most frequently observed. One species, Streptojnyces 

 coelicolor {Actinomyces violaceus-ruber) , shows regularly a very 

 striking color change, being at first red and later a pronounced blue. 

 This has been shown to be due to a change in the reaction of the 

 medium, the pigment being very similar to litmus and changing from 



