1 6? NUTRITION AND METABOLISM 



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 mis- 

 take is quite commonly 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 

 cultivated organisms are not alone in showing this tendency toward 

 variation. The transferring of a soil or water bacterium into the ordi- 

 nary laboratory media is a complete change of conditions; the different 

 cells of the same species may react differently and give several varie- 

 ties. A lactic bacterium on meat medium withoijt sugar does not thrive 

 well in the first generations, but it gradually becomes able to grow on 

 this medium. By this treatment, it loses gradually the power of pro- 

 ducing acid and does not thrive as weU in milk. The attenuation of 

 pathogenic bacteria by cultivation on media, as potato, very different 

 from the blood and muscle upon which they grow most naturally, or 

 by growing them at low temperature, or above the maximum, furnishes 

 another example. The decrease and finally the entire loss of patho- 

 genicity is caused by a change of metabolism, by a loss of the power to 

 produce toxin. 



As by certain diet the metaboUsm can be changed, so certain 

 physiological properties of bacteria can, by proper cultivation, be 

 increased. By the frequent transferring of an organism on gelatin, its 

 liquefyiag quaUties can be increased, provided it had some at the start. 

 By continued passing of a bacterium through an animal, its virulence 

 can be increased. Strains of bacteria which will produce a very high 

 acidity can be bred; this is illustrated by the quick-vinegar process 



