LACTIC ACID BACTERIA. 107 



these several fermentations doubtless represent many different 

 species. 



The lactic acid developed in the fermentation of milk, an 

 acid first definitely characterised by Scheele in 1780, corre- 

 sponds approximately with the quantity of lactose that has 

 disappeared. Only minute quantities of by-products are 

 formed, as was proved by the detailed researches of Kayser. 

 If the fermentation is continued for some time, many species 

 will decompose part of the lactic acid originally formed. 

 Kayser found that a pure cultivated species from cream 

 grown in lactose-peptone-wort had lost 0-26 gramme of lactic 

 acid per litre in eleven days. If volatile fatty acids are formed, 

 they will tend to increase in quantity under these conditions 

 at the expense of the lactic acid. According to 0. Jensen, 

 lactic acid may itself be converted into volatile fatty acids. 



Lactic acid formed by the spontaneous fermentation of 

 milk is usually optically inactive i.e., it does not turn the 

 plane of polarised light either to the right or to the left. If, 

 however, the active bacteria are isolated in pure cultures, 

 and inoculated into sterile milk, species are developed which 

 produce a lactic acid turning the plane of polarised light to 

 the right (dextro-rotatory bacteria), and others producing a 

 laevo-rotatory acid. The dextro-rotatory species occur more 

 frequently. Thus the species of bacteria determines whether 

 one or the other sort of acid shaU be produced. It appears, 

 however, that there are also species of lactic acid organisms 

 in which the optical activity of the product . of fermentation 

 depends upon the composition of the nutritive fluid, as shown 

 especially by Kayser. The species reacts differently with dif- 

 ferent sugars. Thus the common Bact. lactis acidi (Leichmann) 

 ferments dextrose, lactose, maltose, mannite, and raffinose. 

 Hueppe's Bac. acidi lactici ferments saccharose, dextrose, 

 lactose, and mannite. A few species thrive best when they 

 have access to atmospheric oxygen, whilst others carry on the 

 fermentation equally well, or even much better, in the absence 

 of air. They also exhibit differences in the rapidity with 

 which acidification takes place at different temperatures. 

 Thus, the Bact. lactis acidi has its optimum for the formation of 

 acid at 32-38 C., Hueppe's Bac. acidi lactici at 35-42 C., 



