64 Biology of Micro-organisms 



was already present, but that more have been produced by the 

 growth of the bacteria. 



The presence of ammonia in either fluid is easily determined by 

 the immediate development of a yellow color or precipitate when a 

 few drops of Nessler's solution* are added. 



Failure to determine either ammonia or nitrites may not mean that 

 the nitrates were not reduced, but that they were reduced to N. It 

 is, therefore, necessary to test the solutions for nitrates, which is 

 done by the use of phenolsulphonic acid and sodium hydroxid, 

 which in the presence of nitrates give a yellow color. 



Combination of Nitrogen. Not only do bacteria destroy or re- 

 duce nitrogen compounds, but some of them are also able to assimi- 

 late nitrogen from the air and so combine it as to be useful for the 

 nourishment of vegetable and animal life. The most interesting 

 organisms of this kind are found upon the roots of the leguminous 

 plants, peas, clover, etc., and have been studied by Beyerinck.f 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 agriculturist 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 appar- 

 ent change in the milk, some producing coagulation, some gelatiniza- 

 tion 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 possessing 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 similar 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. Be- 

 tween 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 

 determine that a micro-organism is possessed of pathogenic 

 powers, the committee of bacteriologists of the American Public 



* Nessler's solution consists of potassium iodid, 5 grams, dissolved in hot 

 water, 5 cc. Add mercuric chlorid, 2.5 grams, dissolved in 10 cc. of water, 

 then to the mixture add potassium hydrate, 16 grams, dissolved in water, 40 cc. 

 and dilute the whole to 1000 cc. 



f "Centralbl. f. Bakt.," etc., Bd. vn, p. 338. 



