254 NUTRITION AND METABOLISM 



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 without 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 well 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 metabolism 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 

 liquefying qualities 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 

 and by the strong alcohol-producing yeasts of the distillery process. 

 By continued cultivation of an organism upon a certain medium, it 

 will become so acclimatized that it degenerates readily when the con- 

 ditions become unfavorable Such specifically trained strains of 

 microorganisms are used in alcoholic and lactic fermentation, in patho- 

 genic bacteriology and in the inoculation of leguminous plants with 

 nitrogen-fixing bacteria. 



FACTORS INFLUENCING THE TYPE OF DECOMPOSITION 



In the chapter on products of metabolism, it has been shown 

 that the same compound can be decomposed in many different ways, 

 and the question may well be asked what decides the type of decomposi- 

 tion. Since bacteria are widely distributed, it must be expected that 

 there are certain conditions which are most favorable to a given type 

 of fermentation, while under changed conditions, other types are more 

 likely to dominate. The fact that sugar in cider nearly always under- 

 goes alcoholic fermentation, while in milk it undergoes lactic fermen- 



