Chapter IV 



VARIATIONS AND MUTATIONS 



No other branch of biology offers so rich a field for the study of 

 variations and mutations and for the rapid selection of new varieties, 

 as that of microbiology. This is due to the simple fact that many gen- 

 erations of organisms can be obtained in a very short time. The fact 

 that these organisms can be grown in an absolutely pure culture, free 

 from any other organisms, and that the composition of the medium and 

 the conditions of growth are easily controlled are other contributing 

 factors. Among the various groups of microorganisms that are readilv 

 subject to variations and mutations, the actinomvcetes occupy a prom- 

 inent place. 



Actinomycetes are greatly influenced, especially in their cultural 

 characteristics, by the composition of the medium and by the conditions 

 of growth. Variations resulting from cultural differences have often 

 led to expressions of doubt concerning the existence of definite types or 

 species among the actinomycetes (261). Because of this doubt, the use 

 of "species-groups" rather than of definite species for the classification 

 of actinomycetes has been suggested (443). 



The general appearance of the actinomyces colony, the abundance 

 and formation of aerial mycelium, the manner of sporulation, the 

 production and nature of endopigments and exopigments, and the vital- 

 ity of the organism when grown on different media make up the 

 variation complex of actinomycetes. 



Types of Variation:— 



General variations.— Early students of actinomycetes recognized the 

 fact that variations among actinomycetes are of several types. Lieske 

 (260) demonstrated that actinomycetes show greater variability in their 

 morphological and physiological properties than do any other group of 

 microorganisms. He classified the types of variations as (a) simple 

 modifications, (Z?) permanent modifications, and (c) mutations, includ- 

 ing the formation of sectors within a colony. Waksman (446) empha- 

 sized that the variations among actinomycetes differ in quantity and in 

 quality, not only under the influence of various environmental condi- 

 tions but even on continued cultivation under the same conditions. 

 Tjhe soluble pigment may be lost or changed in color; the color of the 

 aerial mycelium may change; even the property of forming aerial 



