220 OUTLINES OF BACTERIOLOGY 



one of the bacteria (Bacterium vermiforme) (Fig. 126). The latter has 

 gelatinous walls, and is sometimes composed of filaments, sometimes 

 of very short rods, and in this jelly-like mass the yeast cells are 

 embedded. Fermentation is set up by the combined activity of these 

 two organisms. As explained in a previous chapter, when two 



organisms live thus amicably 

 together, they are said to be in a 

 state of symbiosis. The turbidity 

 which is noticed during fermenta- 

 tion is due to the budding off into 

 the liquid of large numbers of 

 yeast cells from the "plant," 



FIG. 126.-Section through "Ginger-beer which tllCll multiply and give oft* 



*^^-S%^^*^^ bubbles of the gas carbon dioxide. 



After a time the liquid becomes 



so viscous that the gas bubbles escape with difficulty from the liquid, 

 the viscosity being due not so much to the presence of yeast-cells as to 

 innumerable bits of gelatinous material derived from the bacterium. 

 At this stage, also, myriads of rod-shaped individuals belonging to the 

 bacterial partner are found in the fermenting mass. At the close of 

 fermentation, the solution contains alcohol, acetic acid, and an incom- 

 pletely known acid resembling lactic acid. The amount of proof-spirit 

 present in ginger beer varies from 1 to 5 per cent. 



(e) Koumiss and Kephir. These beverages are produced by the 

 fermentation of milk-sugar or lactose, and- are prepared in Russia and 

 various parts of Central Asia. 



Koumiss is a preparation obtained by fermentation of the milk 

 yielded by mares. The milk is placed in small casks, which then 

 receive lumps of old koumiss used in previous fermentations. A lump 

 of koumiss consists of a mixture of several organisms, intimately 

 blended together, as in the ginger-beer plant. One of these organisms 

 is a yeast, whilst others are lactic-acid bacteria. These feed on the 

 milk-sugar, producing alcohol, carbon dioxide, and lactic acid. When 

 the fermentation is nearly completed, the liquid is placed in bottles, 

 the latter being then fitted with tight-fitting, securely-wired corks. 

 The fermentation is finished inside the bottles, and, as carbon dioxide 

 continues to be given off, an effervescent beverage is obtained. Mare's 

 milk contains about 5 '5 per cent, of sugar, whilst in koumiss the 

 percentage is reduced to 1/3. On the other hand, koumiss contains 1'6 

 per cent, of alcohol, and nearly 1 per cent, of both lactic acid and 

 carbon dioxide. There is reason to believe that other changes also 



