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 cultivation. 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 intro- 

 duced 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 ginger-beer yeast, investigated by WARD (II.), and 

 used in England for making ginger-beer. 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 suitable for the growth of the second species. This 

 preparatory function 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 indispensable or highly favourable to the other organism. 

 This kind of dependence was styled by GARRK (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 characterised as alcoholic fermentation rapidly sets in. The 

 skin of the grape is the habitat of an abundant flora of fungi, which are intro- 

 duced into the must in the operation of pressing. Of these (exceptional instances 

 apart), the organism exciting alcoholic fermentation is the first to develop, 

 because the constitution of the must favours it the most, the result being that 

 the sugar therein contained is split up, and carbon dioxide and alcohol are pro- 

 duced. When this decomposition is effected, the character of the liquid has 

 become changed, and now a new species, exciting acetic fermentation, comes into 

 play. This organism was already present in the must, but could not make head- 

 way against the predominant yeast, because, in the first place, the alcohol, 

 without which it feeds but indifferently, was lacking. Secondly, even had this 

 substance been present, it could not have been utilised, because of the atmosphere 



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