CHAPTER VIII. 



BACTERIA IN THEIR RELATION TO ONE ANOTHER. 



64. Symbiosis, Metabiosis, Antagonism. 



IT is only in exceptional cases that a sample of a natural liquid 

 contains but a single species of micro-organism when in a state of 

 fermentation. Nearly always we have to deal with a mixture of 

 several species, the separation of which one from another, and 

 the reproduction of the isolated individuals, is termed pure cul- 

 tivation. A liquid or solid nutrient medium inhabited by a 

 single species is called a pure culture, the methods of preparing 

 which will be considered in the next section. 



When two or more species are simultaneously engaged in the 

 consumption of a given nutrient medium, their association is termed 

 Symbiosis. A couple of examples will serve to make this clear, 

 one of them being the Kephir granules, which will be described 

 in a later chapter. These granules chiefly contain two classes of 

 organisms, lactic acid bacteria and yeasts ; and when introduced 

 into milk the fission fungi generate acidity, whilst the yeasts 

 decompose a portion of the milk-sugar and produce alcohol and 

 carbon dioxide. In this way an acid, foaming liquor known as 

 " kephir " is obtained. A second, cognate example is afforded by 

 the gingerbeer yeast, investigated by WARD (II.), and used in 

 England for making gingerbeer. This is another instance of 

 symbiosis, viz., the association of Saccharomyces pyriformis with 

 a fission fungus, Bacterium vermiforme, the latter of which as 

 is described in Chapter xxv. induces lactic fermentation in 

 (spiced) cane-sugar solutions. 



The mutual relation of two or more species contained in the 

 same culture may, however, be such that the one species, by the 

 exercise of its vital functions, renders the nutrient medium suit- 

 able for the growth of the second species. This preparatory func- 

 tion of the one species may consist either in the absorption and 

 elimination of certain constituents of the medium which retard 

 the development of the other species, or in the excretion of 

 certain products otherwise lacking in the medium, and either in- 

 dispensable or highly favourable to the other organism. This 

 kind of dependence was styled by GARR] (I.) Metabiosis, an 

 excellent example of which is afforded by the decomposition set 

 up in natural wine-must. If this be allowed to stand in an open 

 vessel as soon as it comes from the press, a decomposition charac- 

 terised as alcoholic fermentation rapidly sets in. The skin of the 



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