34 OFFENSIVE FORCES OF THE INVADING MICROORGANISM 



albumins, such as serum, serum agar, hydrocele agar, milk, etc. 

 When the organism is transplanted to ordinary media this peculiarity 

 rapidly disappears, but can be made to reappear by transferring it 

 to albuminous media, or by reinoculation into the animal body, and 

 so on indefinitely. The most notable organisms which possess this 

 property, aside from the capsule bacteria proper, viz., those organ- 

 isms which even under ordinary circumstances possess a capsule, 

 such as the bacillus pneumonise of Friedlander, and the bacillus of 

 rhinoscleroma, are a number of streptococci (str. involutus, vulvitidis 

 vaccarum, mastitidis vaccarum, equi, mucosus), the pneumococcus, 

 the micrococcus tetragenus, bacterium anthracis, bacterium pestis, 

 bacterium cholerse gallinarum, certain pathogenic yeasts, etc. 



In other organisms actual capsule formation has not been observed 

 but in its place an analogous process has been noted, resulting in 

 a thickening of the ectoplasm, so that the bacteria look larger and 

 coarser. This is true especially of the colon and typhoid bacillus 

 and of the staphylococcus, and leads to appearances which often 

 contrast strongly with the tiny attenuated forms which one is accus- 

 tomed to see in .old cultures, on the ordinary media. 



The importance of these morphological changes as a defensive 

 mechansim of the bacteria against the opposing forces of the host 

 can hardly be overestimated. It has been conclusively demon- 

 strated, as a matter of fact, that such "animalized" bacteria, as 

 Bail terms them, offer a far greater resistance to the destructive 

 action of bactericidal sera and to phagocytosis than do the corre- 

 sponding forms which have been cultivated on the ordinary media. 

 That such changes must of necessity lead to a marked increase 

 in the virulence of an organism is of course self-evident. This 

 is well illustrated by an experiment of Horiuchi, who relates that 

 he had in his possession a highly virulent, densely capsulated strain 

 of the micrococcus tetragenus, which resisted phagocytosis almost 

 entirely and killed guinea-pigs in a dose of 100 organisms. When 

 this was grown for a number of days on rather dry agar it lost its 

 capsule-forming power permanently, became readily subject to 

 phagocytosis, and did not affect guinea-pigs even in doses of one 

 thousand million organisms. 



In view of our present knowledge of the relation between capsule 

 formation and virulence, we can now readily understand why animal 

 passage of an organism leads to increased virulence. This fact had 



