48 OUTLINES OF BACTEEIOLOGY 



platinum needle, and inoculate an agar-nutrient-tube. This is placed 

 in a cupboard at room temperature, or, when growth is slow, in an 

 incubator. In about 15 hours, owing to the bacterial growth, the 

 surface of the agar will be covered by many millions of the organism. 

 They appear to the naked eye as a transparent or opaque and coloured 

 covering, according to the nature of the species. There is a good deal 

 of variation in the nature of this growth. The chief point to note at 

 present is the fact that all these millions of individuals are derived 

 from a single individual, therefore this is a pwre culture. If the study 

 of this microbe is desirable as being of importance in any branch of 

 industry, or the cause of some disease, that study can now be carried 

 out, and the characteristics of the species accurately observed without 

 any complications due to the presence of other bacteria. Our know- 

 ledge of such diseases as consumption, lockjaw, diphtheria, etc., is now 

 widely extended ; and that is due to the fact that the effects produced 

 by the micro-organisms in these diseases have been carefully studied 

 by the aid of pure cultures. Among yeasts, the isolation of the various 

 species, by Hansen and others, has enabled brewers to work with pure 

 cultures when transforming the wort into beer. In working with 

 mixed yeasts a brewer is never certain when a predominance of "wild " 

 yeasts may occur. If they gain the upper hand in wort, the loss to 

 the brewer is considerable. Finally, we may mention the use of pure 

 cultures for the souring of cream, preparatory to churning, in the pro- 

 duction of butter. It is to be hoped that other fermentative industries 

 will, in course of time, be able to utilise pure cultures, instead of, as at 

 present, relying on chance organisms that are present in the atmo- 

 sphere, to produce the desired effects. 



§5. METHODS OF EXAMINATION OF BACTERIA. 



The identiiication of bacteria is not a simple matter, for, first, there 

 are so many organisms with similar appearances and of an approximate 

 size and shape, on the surface of nutrient media ; and, secondly, there 

 is so much plasticity in a bacterial growth that the same species under 

 different conditions can never be relied upon to have the same character- 

 istics in all the cultures. Sarcina mobilis may serve as an instance of 

 this plasticity. In the cultures of Maurea, by whom the species was 

 discovered, the growths were brick-red in colour. Migula, who obtained 

 his culture from Maurea, states that his cultures were orange-yellow. 

 In the author's cultures, obtained indirectly from Migula, the growths 



