284 PATHOGENIC BACTERIA. 



develop upon the plates appear as transparent, delicate, 

 drop-like accumulations, scarcely visible to the naked 

 eye, but under the microscope distinctly granular, the 

 central darker portion being frequently surrounded by a 

 paler marginal zone. 



In gelatin puncture-cultures, made with 15 instead of 

 the usual 10 per cent, of gelatin, the growth takes place 

 along the entire path of the wire in the form of little 

 whitish granules distinctly separated from each other. 

 The growth in gelatin is always very limited. 



Upon agar-agar and blood-serum the growth consists 

 of minute, transparent, semi-confluent, colorless, dew- 

 drop-like colonies, which die before attaining a size 

 which permits of their being seen without careful in- 

 spection. Upon glycerin agar-agar the growth is more 

 luxuriant. 



In bouillon the organisms grow well, clouding the 

 medium very slightly. 



Milk is quite well adapted as a culture-medium, its 

 casein being coagulated. It does not grow upon potato. 1 



When it is desired to maintain or increase the virulence 

 of a culture it must be very frequently passed through 

 the body of a rabbit. The degree to which the virulence 

 can be raised in this way is remarkable. C. W. Lincoln 

 has succeeded in reducing the fatal dose for rabbits to 



ToTTOlfFoTFo °* a c - cm - 



If a small quantity of a pure culture of the virulent 



organism is introduced into a mouse, rabbit, or guinea- 

 pig, the animal dies in one or two days. Exactly the 

 same result can be obtained by the introduction of a 

 piece of the lung-tissue from croupous pneumonia, by 

 the introduction of some of the rusty sputum, and gener- 

 ally by the introduction of saliva. 



The post-mortem shows that an inflammatory change 

 has taken place at the point of inoculation, with a fibrin- 



1 Ortmann asserts that the pneumococcus can be grown on potato at 37 C, 

 but this is not generally confirmed. The usual acid reaction of the potato 

 would indicate that it was a very unsuitable culture-medium. 



