12 ' AIDS TO BACTERIOLOGY 



similar quantities of the reagent and of a saturated aqueous 

 solution of potassium persulphate are added, when indole 

 produces a rose-pink colour. 



The cholera spirillum reduces peptone with formation of 

 nitrites, and gives the reaction on the addition of acid alone. 



The production of indole depends on the presence of the 

 tryptophan group in the culture medium,and Zipf el has pro- 

 posed the following medium as most suitable : ammonium 

 lactate (0-5 per cent.), dicalcium phosphate (0-2 per cent.), 

 magnesium sulphate (0-02 per cent.), and tryptophan 

 (0-03 per cent.). 



Production of Acid and Alkali. Many bacteria produce 

 ammonia in a sugar-free broth, while the power of some 

 to ferment sugars, with the production of acid, and some- 

 times of gas as well, is used for diagnostic purposes. 



Nitrification, Denitrification, Nitrogen Fixation. The 

 bacteria concerned in the nitrogen cycles are dealt with 

 in the chapter on ' The Bacteriology of Soil.' 



The Ptomines (Cadaveric Alkaloids). During the de- 

 composition of proteins, bodies similar in constitution 

 to the vegetable alkaloids may be formed. Some, like 

 methylamine, dimethylamine, and trimethylamine, are 

 non-poisonous; but others, such as muscarin (found in 

 poisonous mushrooms), tyrotoxicon (found in poisonous 

 cheese, milk, and ice-cream), and mytilotoxin (found in 

 mussels), have very toxic properties. It is doubtful 

 whether these products are responsible for so-called pto- 

 mine poisoning. Many outbreaks are due to infection with 

 an organism (such as B. enteritidis). See ' Bacteriology of 

 meat,' Chapter XX. 



Toxins. Bacteria may give rise to disease in various 

 ways: by appropriation of nutriment, by abstraction of 

 oxygen from the tissues, and obstruction of capillary 

 vessels through their excessive multiplication. Their most 

 important method of aggress, however, is in the production 

 of poisonous bodies, known as ' toxins.' Toxins are 

 supposed to be protein in nature, and in many cases re- 

 semble enzymes. They are probably not products of 

 disintegration, like the ptomines, but are specific meta- 

 bolic products of the bacterial cell. 



Toxins may be retained in the bacterial cell as an 

 integral part thereof (endotoxins or intracellular toxins), 

 or, after their formation in the cell, they may be excreted 



