Production of Disease 57 



14. Production of Disease. Bacteria that produce 

 disease are known as pathogenic; those that do not, as 

 non-pathogenic. Between the two groups there is no sharp 

 line of separation, for true pathogens may be cultivated 

 under such adverse conditions that their virulence may be 

 entirely lost, while bacteria ordinarily harmless may be 

 made virulent by certain manipulations. In order to 

 determine that a micro-organism is possessed of pathogenic 

 powers, the committee of bacteriologists of the American 

 Public Health Association* recommends that : (i) When a 

 given form grows only at or below i8-2O C., inoculation 

 of about i per cent, of the body- weight with a liquid culture 

 seven days old should be made into the dorsal lymph-sac 

 of a frog. (2) When a species grows at 25 C. and upward, 

 an inoculation should be made into the peritoneal cavity 

 of the most susceptible (in general) of warm-blooded animals 

 i. e., the mouse, either the white or the ordinary house 

 mouse. The inoculation should consist of about i per cent, 

 of the body-weight of the mouse of a four- to eight-hour 

 standard bouillon culture, or a broth or water suspension 

 of one platinum loop from solid cultures. When such 

 intraperitoneal injection fails, it is unlikely that other 

 methods of inoculation will be successful in causing the 

 death of the mouse. If the inoculations of the frog and 

 mouse both prove negative, the committee think it un- 

 necessary to insist upon any further tests of pathogenesis 

 as being requisite for work in species differentiation. 



Production of Enzymes by Bacteria. Some of these 

 have already been mentioned as causing fermentation and 

 putrefaction, coagulating milk, dissolving gelatin, etc. 

 There are, however, others which have interesting and im- 

 portant actions upon both animal and vegetable substances. 



Knowledge upon the subject is just becoming systema- 

 tized, one of the best writings being by Emmerich and 

 L6w,f who observed that in old cultures of Bacillus pyo- 

 cyaneus the bacteria become transformed into a gelatinous 

 mass, and were led to experiment with old and degener- 

 ating cultures condensed to ^ volume in a vacuum appa- 

 ratus. The bacteriolytic powers were then found to be 

 much increased, and they were subsequently able to precipi- 

 tate from the concentrated culture an enzyme, which they 



* "Jour. Amer. Public Health Assoc.," Jan., 1898. 

 f "Zeitschrift fur Hygiene," 1899. 



