MODIFIED MEDIA. 241 



ingly well on glycerinated media. The manner in which 

 glycerin favors the growth of the tubercle bacillus is not 

 known, but inasmuch as this organism stores up within its 

 cell a large amount of fat, it would seem as if the glycerin 

 was directly utilized for the making' of this compound. 



Glucose media. The addition of 2 per cent, of glucose to 

 bouillon, gelatin, or agar greatly improves these as media 

 for the cultivation of anaerobic bacteria. Glucose is a pow- 

 erful reducing 1 substance, taking" up oxygen from its neigh- 

 borhood. In this way, it is supposed to favor the growth of 

 these bacteria. Again, certain bacteria readily ferment 

 glucose, giving- rise to acid, and to gaseous products. Hence, 

 the addition of glucose is of great value in differentiating 

 species. Sometimes litmus is added to glucose media in or- 

 der to indicate the reaction produced by the organism. Lit- 

 mus glucose gelatin is especially useful for cultivating 

 anaerobic bacteria, with or without a special apparatus. 

 Moreover, the vitality of these organisms are greatly pro- 

 longed in this medium. 



Lactose media. Instead of glucose, 2 per cent, of milk- 

 sugar may be added to the several media. Usually, the lac- 

 tose media are colored at the same time with litmus. Cer- 

 tain bacteria, like the typhoid bacillus, cannot ferment lac- 

 tose, and hence the reaction remains alkaline and no gas 

 bubbles are given off. On the other hand, some bacteria 

 like the colon bacillus, do ferment lactose, in which case the 

 litmus turns red and gas bubbles appear. Lactose media 

 are therefore valuable as a means of differentiating organ- 

 isms. 



Litmus media. The commercial litmus usually contains 

 a large admixture of red pigment which interferes, more or 

 less, with the delicacy of the reaction. It is advisable, 

 therefore, to purify the litmus. This can be done according 



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