160 CULTURE MEDIA 



mined best by titration with twentieth-normal sodium 

 hydroxide solution with phenolphthalein as indicator. The 

 amount of normal alkali or acid as found by titration is now 

 added to the medium to make the desired end reaction (most 

 commonly 1 to 1.5 per cent, acid to phenolphthalein) and the 

 medium is again thoroughly boiled and filtered boiling hot. 

 The titration and boiling ordinarily cause a precipitate to 

 form which is largely phosphates of the alkaline earths with 

 some protein. The filtered medium is collected in suitable 

 containers, flasks or tubes, which are plugged with well- 

 fitting non-absorbent cotton plugs and sterilized, best in the 

 autoclave for twenty minutes at 15 pounds' pressure, or discon- 

 tinuously in streaming steam at 100°. If careful attention is 

 paid to titration and to sufficient boiling where indicated, the 

 meat broth prepared as above should be clear, only faintly 

 yellowish in color and show no precipitate on cooling. 



Broth may be prepared from Liebig's or Armour's meat 

 extract by adding 5 grams of either, 10 grams peptone and 



5 grams NaCl to 1000 c.c. of water, boiling to dissolve, then 

 titrating and filtering as above. 



The author after much experience finds meat juice prefer- 

 able to meat extract for broth and other media for patho- 

 genic bacteria, and has abandoned the use of meat extracts 

 for these organisms. 



Glycerin Broth. — Glycerin broth is made by adding 4 to 



6 per cent, of glycerin to the broth just previous to the 

 sterilization. The glycerin serves as a source of carbon to 

 certain bacteria which will not grow on the ordinary broth — 

 as Bacierium tuberculosis. 



Sugar Broths. — Sugar broths are used for determining the 

 action of bacteria on these carbohydrates, since this is a, 

 valuable means of differentiating certain forms, especially 

 those from the intestinal tract. Broth free from sugar must 

 first be made. This is done by adding to broth prepared as 

 already described, just previous to final fiUeriug and steriliza- 

 tion, a culture of some sugar-destroying organism {Bacillus 

 coli is ordinarily used), and then allowing the organism to 

 grow in the raw broth at body temperature for twenty-four 

 hours. Any carbohydrate in the broth is destroyed by the 



