THE LACTIC-ACID BACTERIA 471 



water. This souring of milk is due to the accumulation of lactic acid, 

 which is formed from the lactose, or milk sugar, of the milk. This 

 change is brought about by a great variety of bacteria, which, broadly 

 speaking, are always present in milk, and which, in their relation to 

 milk, are known under the collective name of lactic-acid bacteria. 

 The grouping is entirely arbitrary and artificial, because the organ- 

 isms belong to various types and have in common only the property 

 of splitting up lactose and forming from it lactic acid. 



The change of lactose into lactic acid is chemically a process of 

 hydrolysis, i. e., a chemical change in which water is added to a 

 molecule, and this molecule subsequently becomes split up into other 

 compounds. The change is probably due to a so-called soluble 

 ferment or enzyme, secreted by the lactic acid bacteria or contained 

 in their bodies, where it may act upon the lactose which diffuses into 

 the substance of the bacterium by osmotic processes. The chemistry 

 of the process is expressed by the following formula : 



+ H 2 = 2C 6 H 12 B = 4C 3 H 6 3 



Lactose + Water = 1 Glucose and = 4 Lactic 

 1 Galactose acid 



In order to understand the names given to some of the bacteria of 

 the lactic-acid group it is necessary to know that lactic acid is a 

 stereoisomeric body. This term means a body or chemical substance 

 existing in nature in two forms of crystallization, which bear the 

 relation to each other of a physical object to its image in a plane 

 mirror and which, when in solution, will behave in the following 

 peculiar manner toward rays of polarized light: One form of crystals, 

 or bodies in solution, will deflect or deviate the polarized rays of light 

 from their straight path toward the right side, and these are called 

 dextrogyr; the other form will deviate or deflect the polarized rays 

 of light from their straight path toward the left side, and these are 

 called sinistrogyr, or levogyr. By a mixture of these two forms a 

 solution may be obtained which will deflect the polarized light, 

 neither to the right nor to the left side. 



THE LACTIC-ACID BACTERIA. 



The bacteria most commonly found in milk and producing in it 

 the most rapid and obvious changes are those which possess the 

 power to ferment milk sugar (lactose) and form from it lactic acid. 

 They are known collectively as the lactic-acid bacteria and occur 

 very widespread in nature. They have been found in hay and straw 

 (Leichmann, Gruber, and others), also in the dust in barns and other 

 places, on ordinary grasses and cereals, and other cultivated plants. 

 Beyerinck found them in the feces of man and animals, and Barthel 

 has found them practically wherever man, animals, and cultivated 



