PREPARATION OF PUKE CULTURE. 45 



and treating the whole growth, without condescending to 

 isolate single cells. 



The physiological methods " the enriching process " 

 employed by Pasteur, Cohn, and others start with the 

 fundamental idea that the various species occurring in a 

 mixture will multiply unequally according to their different 

 natures, when they are cultivated in one and the same 

 nutritive liquid and at the same temperature, so that those 

 species for which the conditions are unfavourable will 

 be gradually suppressed by the one or more species for 

 which the conditions are favourable. When the growth has 

 developed under the selected conditions for quite a short 

 time, a minute fraction is inoculated into the same nutritive 

 liquid in a fresh vessel at the same temperature, and this 

 process is repeated many times. Different liquids have been 

 employed for such cultures from time to time ; for instance. 

 alkaline liquids for bacterial growths, acid liquids to free 

 yeast growths from bacteria (lactic, tartaric, hydrofluoric acids, 

 etc.). The weak point of all such methods is, that they start 

 from an unknown material namely, the impure mixture. It, 

 is, therefore, impossible to know what results such a treatment 

 will lead to, for we are not dealing with any true method, as 

 contamination may take place at random. In fact, there is 

 always the possibility that the weaker species are not destroyed, 

 but merely checked and retarded, so that when the stronger 

 species, after reaching their maximum development, become 

 weaker, other species will have a chance of multiplying. 

 This possibility also occurs when the growth is transferred 

 to another substratum. Likewise, there is always the possi- 

 bility that not one but two or more species thrive equally 

 well in the liquid, and, consequently, develop to the same 

 extent. Such, for instance, was the case with brewers' yeast 

 before pure cultures were employed. This yeast often yielded 

 several typically different speoies of " culture yeast," as they 

 are termed, when examined by Hanson's method. Th^ 

 method given by Pasteur for the puri.lcation of brewers' 

 yeast may be mentioned as a marked illustration of the dangers 

 connected with the physiological method of treatment. TL 

 impure yeast-mass is introduced into a can^-sugar solution. 



