148 MICROBIOLOGY 



milk which has been seterilized at 55 or 60 C. It has been 

 asserted that these ferments may be present in all species of 

 bacteria which coagulate milk, even though the organisms also 

 act upon the sugars with the production of acid. 



Fermentation yields products that inhibit the action of 

 the ferments. Fermentation ceases when it reaches a certain 

 point. Different kinds of fermentation are named according 

 to the products. Thus acetic fermentation yields acetic acid ; 

 alcoholic or vinous fermentation produces alcohol ; ammoni- 

 acal yields ammonia ; amylic yields amylic alcohol ; butyric 

 yields butyric acid; lactic yields lactic acid; and the like. 



Aerobic bacteria produce alkaline products from al- 

 buminous substances in culture media free from sugar. Many 

 species of bacteria produce acids in the presence of sugars, 

 which explains the fact that neutral or slightly alkaline broth 

 often becomes acid in young cultures from the fermentation 

 of the sugar contained in the meat used for making the 

 media. When the sugar is used up the reaction often becomes 

 alkaline due to the action on the proteid. The substances 

 producing the alkalinity in cultures are chiefly ammonia, 

 amine, and the ammonium bases. 



The conversion of urea into the carbonate of ammonia 

 affords an example of the production of alkaline substances by 

 bacteria : 



CO(NH 2 ) 2 + 2H 2 0--=C0 2 (NH 4 ) 2 . 



Urea 2 water ammonium carbonate. 



Ptomains. Nencki, Brieger, Vaughan and others have 

 succeeded in obtaining organic bases of a definite chemical 

 composition out of putrefying substances such as meat, fish, 

 old cheese and milk. Some of these were found to exert a 

 poisonous effect upon animals, while others were harmless. 

 The ptomains may be present in the decomposing animal body 

 hence the name ptomain. They may be formed also in the 

 living animal body, and, if not made harmless by oxidation, 

 may act as self -poisons or leucomains. The ptomains are de- 

 scribed by Vaughan as cleavage products. 



