148 BACTERIA. 



As another example of a facultative pathogenic parasite, 

 the anthrax bacillus may be cited. It may develop actually 

 in the tissues, as in woolsorters' disease, where it occurs in 

 the bronchial mucous membrane ; in the lymphatic tissue 

 spaces, and in the pleural cavities ; or in the blood, as in 

 cases of accidental inoculation. It may also occur in the 

 blood in woolsorters' disease, the bacillus readily making its 

 way from the air vesicles and tubes into the pulmonary 

 circulation. In this connection it may be mentioned that 

 anthrax bacillus cannot undergo its whole developmental 

 cycle within the body ; the vegetative stages only having 

 been found in parasitic anthrax ; whilst growing on^ dead 

 tissues at a favourable temperature spore formation is 

 invariably noticed at some period or other during the life 

 history of the anthrax rods, so that the bacillus must be 

 looked upon as a facultative saprophyte. Another point to 

 which attention may be drawn is that bacteria must be 

 looked upon as essentially and primarily saprophytic organ- 

 isms. By a process of long and gradual acclimatization or 

 adaptation, certain species have become so altered, either 

 temporarily or permanently, that they are able to exist as 

 parasites. It will always be found, however, that the 

 tendency to revert to the saprophytic or non-pathogenic 

 condition is more marked than their tendency to become 

 transformed into the parasitic and pathogenic condition. It 

 has been found, for example, that anthrax bacillus passed 

 through a series of cultivations in gelatine, or hog-cholera 

 bacillus " similarly treated, loses a great deal of its patho- 

 genic power. This loss of pathogenic power may take 

 plac^ in two directions, either by the organism losing its 

 power of development in living tissues, or by losing its 

 virulent specific poison. The cholera organism cultivated 

 outside the body usually loses much of its pathogenic power. 

 By appropriate treatment, such as cultivating at the body 

 temperature in specially prepared broth through a number 

 of generations, this pathogenic activity may, however, be 

 restored. We must therefore look upon pathogenicity, para- 

 siticism, and saprophyticism as mere relative terms ; the 

 conditions necessary for the development of the saprophytic 

 mode of life being more widely met with than are those 

 in which the parasitic life may become developed, most 

 organisms become more or less permanently adapted to 



