PRODUCTS OF METABOLISM. 105 



species may react differently and give several varieties. A lactic bac- 

 terium on meat media 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 producing 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 tem- 

 perature, or above the maximum, furnishes another example. The 

 decrease and finally the entire loss of pathogenicity 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 physio- 

 logical 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 il- 

 lustrated by the quick-vinegar process and by the strong alcohol-produc- 

 ing yeasts of the distillery process. It is also possible to accustom micro- 

 organisms to certain chemicals which are antiseptic for the ordinary 

 strains. Distillery yeast is by careful breeding acclimatized to tolerate 

 a very high degree of acidity which would kill an ordinarily bred yeast. 

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

 come so acclimatized that it degenerates readily when the conditions become 

 unfavorable. Such high-bred strains of microorganisms are used in 

 alcoholic and lactic fermentation, in pathogenic bacteriology and in the 

 inoculation of leguminous plants with nitrogen-fixing bacteria. 



PRODUCTS FROM NITROGEN-FREE COMPOUNDS. 



The decomposition products of nitrogen-free organic compounds will 

 be taken up in the same order as was done with the food substances, 

 beginning with the most complex carbohydrates. 



CELLULOSE is attacked by many molds, but little is known about the 

 products. Probably they oxidize it completely to carbon dioxide and 

 water. 



(C 6 H 10 O 5 )n+i2nO-6nCO 2 -H5nH 2 O 



Some bacteria, as B. ferrugineus (van Iterson) and a few denitrifying 

 bacteria, can use cellulose as organic food material; cellulose in soils is 



