60 PATHOGENIC BACTERIA. 



albumins, or from the presence of metabolic products of 

 the bacteria, but also from the irritating bacteria them- 

 selves. It is only in unusually warm weather that these 

 bacteria are able to grow luxuriantly, hence enterocolitis 

 is most common in summer. 



Sometimes the properties of coagulation and digestion 

 of milk are valuable aids in the separation of different 

 species of bacteria. Thus, the colon bacillus coagulates 

 milk, but the typhoid bacillus does not. 



13. Production of Disease. — Bacteria which produce 

 diseases are known as pathogenic ; those which do not, 

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

 sharp line of separation, for true pathogens may be culti- 

 vated under such adverse conditions that their virulence 

 will be entirely lost, while at times bacteria ordinarily 

 harmless may be made virulent by certain manipulations 

 or by introducing them into animals in certain combina- 

 tions. In order to determine that a micro-organism is 

 possessed of pathogenic powers the committee of bacte- 

 riologists of the American Public Health Association 

 recommended that: 1. When a given form grows only at 

 or below i8°-20° C. inoculation of about 1 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 inocu- 

 lation should be made into the peritoneal cavity of the 

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

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

 mouse. The inoculation should consist of about 1 per 

 cent, of the body-weight of the mouse of a 4-8-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. 



