NAHIATIONS. MITATIOXS, AND ADAPTATION'S 



99 



Figure 4S. \';ui;il>ility of aclinoinyccic coioiiics in a jjhite culture i Hc^produccci from: Stanier, R. Y. 

 J. Bacteriol. U: 557. 1942). 



amenable to the environment; (b) continu- 

 ous or fluctuating variations; and (c) devel- 

 opmental variations, resulting in saltations 

 or mutations. The adaptive type is usually 

 characterized by a decrease in the size of 

 the colony, loss of the capacity to form 

 aerial hyphae, reduction in ability to utilize 

 certain nutrients, change in pigment forma- 

 tion, and loss or gain in capacity to produce 

 specific antibiotic .substances. The continu- 

 ous type of variation is marked by the nature 

 and intensity of the pigment formed by the 

 organism, as well as by the capacity to pro- 

 duce a given antibiotic. The developmental 

 variations are also illustrated by the pres- 

 ence or absence of aerial mycelium, pigmen- 

 tation, and production of antibiotics. Some 

 of these changes can be reversed to the orig- 

 inal by growing the organi.sm on special 

 media, such as glycerol mitrient agar or 

 sterile soil. 



Other \'ariatioiis or nuitatioiis are more 

 nearly permanent or more stable in nature, 

 although they may appear only on rare occa- 

 sions. Examples of permanent variations are 



loss of acid-fastne.ss, of pigmentation, and of 

 the ability to form spores. 



The actinomycetes are markedly sensitive 

 to their environment : the extent of mycelium 

 formation can be influenced by a change in 

 the composition of the medium. One of 

 Lieske's cultures produced a well-developed, 

 extensi\-ely branched mycelium in potato 

 extract; it grew in the form of short, coccus- 

 like chains on nutrient agar; and gave ri.se 

 to short, sometimes branched, rods in meat 

 extract-peptone bouillon. Waksman found 

 that in the ca.se of S. reticuli, the sporog- 

 enous hyphae formed verticils on synthetic 

 agar, but showed racemose branching on 

 nutrient agar or on certain inorganic media. 



Several forms of hereditary variation 

 among actinomycetes may be listed: (a) 

 transformation of an actinomycete into a 

 mycobacterium-like organism; (b) trans- 

 formation of an actinomycete into a diph- 

 theroid organism; (c) transformation of 

 anaerobic, short-hyphal-producing forms of 

 actinomycetes into aerobic, long-hyphal 

 forms; (d) change of antibiotic-producing 



