158 BACTERIA AND FERMENTATION 



certain of the compounds that make up the body. In some cases 

 these simpler products that result from the activity of bacteria 

 are very poisonous and in disease it is these substances, toxins, 

 that often cause, the principal danger. Let us now consider the 

 more important of these operations. 



1. Fermentation. — A very large group of bacteria, in common 

 with some fungi, live upon sugars and other carbohydrates, 

 causing a decomposition known as fermentation. Vinegar is 

 due to the decomposition of the alcohol in cider and other weak 

 alcoholic solution by numerous species of bacteria which appear 

 as slimy masses and are popularly known as mother of vinegar. 

 Milk sours and coagulates through the agency of bacteria which 

 reduce the sugar to lactic acid, which in turn coagulates casein. 

 The beverages^ known as zoolack, kumiss, etc., are milk prepara- 

 tions soured by special kinds of bacteria or yeast. The Metch- 

 nikoff tablets contain bacteria that are especially active in this 

 way and they are also supposed to live in the lower alimentary 

 tract where they form decomposition products that are injurious 

 to the harmful bacteria that live in this same region. So also 

 butyric acid, necessary in the manufacture of cheese, is produced 

 by bacteria. It will be seen from this that the products of de- 

 composition are not necessarily harmful. It is interesting to 

 mote that some of the flavors of cheese and of high grade butter 

 are due to the products of decomposition and the excretions from 

 ^bacteria. It is evident that this might be true in the case of 

 limburger cheese and rancid butter but also remember that species 

 of bacteria are cultivated and used to impart certain flavors to 

 cheese and also to a less extent to butter. It is altogether 

 probable that this will become a common practice in our dairies. 



2. Decay. — The great majority of bacteria live as saprophytes 

 upon dead plants and animals. They begin their work of 

 decomposition- immediately upon the death of the organism. 

 In the case of plants they are generally assisted at first by 

 various fungi. There are numerous classes in this group, each 

 kind doing a special work in bringing about the decay. The 

 first steps in the decomposition are quickly performed and then 

 the simpler compounds, though mostly still quite complex, are 



