104 NUTRITION AND METABOLISM. 



exactly decompositions which are caused by mixed cultures. The 

 complications become so great and the relations between different 

 organisms are so little known that it is useless to make the attempt. 



PHYSIOLOGICAL VARIATIONS. 



The great variability of microorganisms in morphological respects has 

 already been pointed out in Part I of this book. A similar variation and 

 adaptation are noticed in their physiology, especially with the food sub- 

 stances of bacteria and consequently with their metabolic products. 

 Microorganisms change their physiological properties very readily with 

 the environment; the new variety may keep its acquired properties for 

 some time even if brought back to the original conditions. It is stated 

 frequently that microorganisms tend more toward variations than the 

 more complex organisms. It should be considered, however, that the 

 experiences in the variations of green plants and animals are based on 

 individuals, while in the case of microorganisms these experiences are 

 gained almost always from millions of cells. A simple illustration is the 

 development of bacteria in salt solutions. If a broth culture of B. coli 

 is transferred into broth containing 8 per cent of salt, a large number of 

 cells will die, often more than 99 per cent. The surviving bacteria begin 

 to multiply after a certain length of time and a new variety is created 

 which can tolerate the salt. At first, only about one out of one hundred 

 cells had the power to tolerate salt, but, since the dying cells are not usually 

 counted or considered at all, it is customary to say that bacteria easily 

 adapt themselves to an 8 per cent salt solution. If only one single plant 

 out of one hundred could be adapted to a certain high temperature, it 

 could not be said that it adapts itself easily. This mistake is quite com- 

 monly made with microorganisms. 



The best illustration for the variability of cultivated microorganisms 

 is the enormous number of varieties of Saccharomyces cerevisia. Nearly 

 every large brewery has a yeast type of its own which differs from others 

 by the amount of alcohol and aromatic substances produced, by time and 

 optimum temperature of spore-production, by the appearance of the 

 budding yeast in the hanging drop, and also in other respects. The cul- 

 tivated organisms are not alone in showing this tendency toward variation. 

 The transferring of a soil or water bacterium into the ordinary laboratory 

 media is a complete change of conditions; the different cells of the same 



