THE BACTERIOPHAGE AND THE BACTERIUM 89 



factors which originated in the struggle which took place between 

 the ancestors of this bacterium and the bacteriophage within the 

 body of the infected animal. As is the case of the resistance 

 acquired experimentally, this naturally acquired resistance disap- 

 pears gradually with repeated culture on laboratory media. Thus, 

 different strains of a single species of bacteria tend to become, 

 by a gradual process, uniform, and after a sufficient number of 

 transplantations all are equally sensitive to the action of the 

 bacteriophage. 



It is worthy of note that the degree of virulence of a bacterium 

 is strictly in relation with its degree of resistance to the bacterio- 

 phage. We will have occasion to demonstrate this frequently 

 when we come to consider the relation between the bacteriophage 

 and the infections due to bacteria. 



MICROSCOPIC OBSERVATIONS 



In the agglutinated and the zoogleic colonies certain atypical 

 forms are encountered, which are also to be found, although less 

 abundantly, in mixed cultures in general. 



Microscopic examination of preparations stained with thionin 

 or by the Giemsa method shows a variety of forms depending 

 somewhat upon the age of the colonies. Up to the second or third 

 day the pleomorphism is considerable. Together with the typical 

 bacillary forms and the cocci there are elongated bacilli, some as 

 long as fifteen micra, with all intermediary lengths; some straight, 

 others curved at all augles. Some are clubbed, yeast-like. Large 

 and small granules are present together with the de"bris derived 

 from the destruction of the different forms. All stages inter- 

 mediary between intact forms and amorphous debris are repre- 

 sented. What is the exact significance of these various forms? 

 They are forms of involution or forms assumed by the bacterium 

 in the development of resistance. That is all that it is possible 

 to say with certainty. However, some of these forms give 

 the impression that the organism in question is reproducing by 

 sporogony. 



In old colonies but very few of the bacillary and coccoid forms 

 are to be seen. 



