34 BIOLOGY OF BACTERIA. 



ferments, diastatic ferments, inverting ferments, and rennet 

 ferments. Other varieties cause lactic, acetic, and butyric 

 acid fermentations. The well-known alcoholic fermentation 

 is not due to bacteria, as was supposed at one time, but to one 

 of the yeast fungi. Fermentation always occurs in carbo- 

 hydrates. 



Putrefaction : This is analogous to the fermentation of the 

 nitrogens. The albumins are converted into peptones, which 

 finally are split up into acids, bases, and salts. Putrefying sub- 

 stances give off a very foul odor. Putrefaction occurs only 

 when the supply of oxygen is deficient for proper combustion 

 in the tissues. Ptomaines, indol, skatol, cadaverin, phos- 

 phoretted hydrogen, ammonium sulphide, and valerianic acid 

 are putrefactive products. Ice-cream and cheese-poisoning 

 are due to a poisonous substance known as tyrotoxicon, which 

 is produced by the putrefaction of the milk proteids before 

 the cream or cheese is made. Other examples are botulismus, 

 or meat-poisoning, and fish-poisoning. 



Acids and alkalies : Acids are formed by bacterial action 

 on the sugar contained in the media. Butyric, acetic, and 

 lactic acids are examples. Ethyl alcohol, aldehydes, and 

 acetone are produced in less quantity. As the formation of 

 the acid continues, the growth of the bacterium is checked 

 and finally inhibited completely. By adding a little blue 

 litmus solution to sterile milk, acid-production is manifested 

 by the changing of the blue color to red. Rosolic acid changes 

 the red to orange. When alkalies are formed, they usually 

 combine with the acids to form salts, and it is more difficult 

 to detect them than the acids. The beef-extract used in the 

 preparation of culture-media contains a slight amount of 

 grape-sugar. 



Reduction of nitrates: The nitrifying bacteria form nitrites 

 from nitrates, nitrogen, and ammonia. Others form nitrates 

 directly from nitrogen and ammonia ; and still another group, 

 which is found in the soil, oxidizes ammonia into nitrites and 

 these into nitrates. This nitrification is of considerable 

 importance to the horticulturist. Some bacteria assimilate 

 nitrogen and combine it so as to furnish nourishment for 

 animals and vegetables. Sterile ground is fertilized in this 



