290 PRINCIPLES OF SOIL MICROBIOLOGY 



particularly with the Hyphomycetes which is a rather loose conglomer- 

 ate of various forms. They should be looked upon as a group of fungi, 

 to be classified separately from the other groups, till their exact syste- 

 matic position has been definitely established. However, the property 

 of acid-fastness correlated with a certain type of pathogenicity (forma- 

 tion of tubercles) and with cross immunity reactions with the acid fast 

 bacteria, points to a certain relation, of at least some pathogenic forms, 

 to the bacteria. 



Species differentiation. Large numbers of actinomyces have been de- 

 scribed by various investigators. 9,17 The larger the number of forms 

 collected, the more difficult is the division into species. The variability 

 of these organisms is such that out of a dozen isolations from a plate 

 not more than two or three may be alike. Even forms recognized to be 

 alike on one medium will be found to be different when grown on another 

 medium. Sometimes two forms found to be alike on several media, 

 will be found to show distinctive characters on further study. 18 This 

 is especially true of the cultural and biochemical characters and to some 

 extent even of the morphology of the organisms. 



It is important first to obtain an absolutely pure culture of the organ- 

 ism. Even then, an attempt to designate a species of actinomyces 

 by the sum total of its morphological and physiological characters 

 may not give very satisfactory results. It is best to classify the organ- 

 isms into groups, defined by a sum total of certain definite morphological 

 and physiological characters. The amount of variability within the 

 group and the amount of overlapping between two groups is something 

 that cannot be definitely established and must be left to the judgment of 

 the investigator. 



The species differ primarily in the length of the mycelium, type of 

 aerial mycelium, absence or presence of spores, method of spore forma- 

 tion, shape and color of colony, formation of soluble pigment, oxygen 

 requirement, production of diastatic and proteolytic enzymes and a 

 number of other morphological and physiological characters. These 

 vary in quantity as well as quality, not only under the influence of various 

 environmental conditions but even on continued cultivation under the 

 same conditions. Not only the soluble pigment may be lost or changed 

 in color, but the color of the aerial mycelium may change and even the 

 very property of forming aerial mycelium may be lost. 



An important advance in the study of this group has been the intro- 

 duction of synthetic media. A great deal of the variability mentioned 



18 Conn, H. J. The use of various culture media in characterizing acti- 

 nomycetes. N. Y. Agr. Exp. Sta. Tech. Bui. 83, 1921. 



