PRODUCTS OF THE GROWTH OF BACTERIA. 129 



I familiar example of this process. The property of liquefying 



? gelatin, or the failure to do so, is used in classifying bacteria 

 and in determining the nature of unknown species. 



Some bacteria, as the bacillus coli communis, form ferments 

 which act like rennet in coagulating milk. Other bacteria are 



i capable of forming sugar from starch. Others have the power 



| of changing cane -sugar into glucose. 



Bacteria which are able to decompose cellulose are found in 



j the stomachs of ruminant animals. Although it is doubtful 

 whether the products of cellulose decomposition have any 

 nutritive value, the process is probably useful in effecting a sub- 

 division of the coarse food, consisting of grass, hay and the like. 

 Some bacteria have the power of decomposing neutral fats 

 into fatty acids and glycerin, after the manner of the fat-split- 

 ting ferment of the pancreatic juice. 



The end-products which result from the growth of bacteria 

 upon albuminous nutrient media are very numerous. They 

 are complicated and not well understood. Among these end- 

 products may be mentioned peptone, indol, skatol, phenol, 

 leucin and tyrosin. Nearly related are the toxins (see Chapter 

 VI.), which play an important -part in the production of dis- 

 ease by pathogenic bacteria. In the decomposition of urine 

 by bacteria the urea is converted into ammonium carbonate. 

 The formation of indol in cultures is an important peculiarity 

 of certain bacteria. The manner of making the test for indol 

 is somewhat differently described by different authors, but the 

 Committee on Standard Methods of Water Analysis, American 

 Public Health Association,* recommend the following: 



The organism to be tested must be made in broth from 

 which all traces of muscle sugar have been removed or in 

 peptone broth. The cultures must be incubated at 37 C. for 

 four days. In applying the test two drops of concentrated 



^Journal 0} Injections Diseases. Supplement No. i, May, 1905, p. 115. 

 See also in this connection Tobey. Journ. Med. Research. XV., 1906, p. 301. 



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