76 Biology of Micro-organisms 



kind are found upon the roots of the leguminous plants, 

 peas, clover, etc., and have been studied by Beyerinck.* 

 It seems to be by the entrance of these bacteria into their 

 roots that the plants are able to assimilate nitrogen from 

 the atmosphere and enrich sterile ground. Every agri- 

 culturist knows how sterile soil is improved by turning 

 under one or two crops of clover with the plough. 



Peptonization of Milk. Numerous bacteria possess the 

 power of digesting peptonizing the casein of milk. The 

 process varies with different bacteria, some digesting the 

 casein without any apparent change in the milk, some pro- 

 ducing coagulation, some gelatinization of the fluid. In 

 some cases the digestion of the casein is so complete as to 

 transform the milk into a transparent watery fluid. 



Milk invariably contains large numbers of bacteria, that 

 enter it from the dust of the dairy, many of them pos- 

 sessing this power and ultimately spoiling the milk. In 

 the process of peptonization the milk may become bitter, 

 but need not change its original reaction. 



The phenomena of coagulation and digestion of milk can 

 be made practical use of to aid in the separation of simi- 

 lar species of bacteria. Thus, the colon bacillus coagulates 

 milk, but the typhoid bacillus does not. 



Production of Disease. Micro-organisms 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 those ordinarily harmless may be made 

 virulent by certain manipulations. In order to deter- 

 mine that a micro-organism is possessed of pathogenic 

 powers, the committee of bacteriologists of the American 

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

 given form grows only at or below 18 to 20 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. 



* See "Centralbl. f. Bakt.," etc., Bd. vii, p. 338. 

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



