12 BACTERIA 



Beggiatoa, Leptothrix, Cladothrix, and, at the top, the 

 Streptothrix. It has been demonstrated that Streptothrix 

 actinomycotica and Streptothrix madurce are the organismal 

 cause, respectively, of Actinomycosis and Madura-foot, two 

 diseases which have hitherto been obscure. 



Pleomorphism. This term designates an irregular develop- 

 ment of a species. Different media and external conditions 

 bring about in protoplasm as susceptible as mycoprotein a 

 variety of morphological phases. These may occur in suc- 

 cession, and represent different stages in the life-history of 

 a bacterium, or they may be involution forms resulting from 

 a change of environment, and occurring as " faults " in the 

 species. In the Bacillus coli, B. typhosus, bacillus of Plague, 

 and B. tuberculosis pleomorphism undoubtedly occurs, and 

 is manifest in the change of shape. This is particularly 

 marked in old cultures of the last named. The ordinary 

 well-known bacillus may grow out into threads, with bul- 

 bous endings, granular filaments, drumsticks, and diplococcal 

 forms. Speaking generally, the older the culture, the more 

 marked is the variation. 



Polymorphism is a term used to define the theory which 

 held that bacteria were one of the intermediate shapes or 

 forms between something lower and something higher in the 

 vegetable kingdom. Neither pleomorphism nor polymor-' 

 phism is fully understood, and many bacteriologists find 

 shelter from both in the term involution form. What we do 

 know is that the species already named, for example, take on 

 divers forms when placed under different conditions. 



Composition. From what we have seen of the diet of micro- 

 organisms, we shall conclude that in some form or other they 

 contain the elements nitrogen, carbon, and hydrogen. All 

 three substances are combined in the mycoprotein or proto- 

 plasm of which the body of the microbe consists. This is 

 generally homogeneous, and there is no sign of a nucleus. 

 It possesses a fortunate affinity for aniline dyes, and by this 



