CULTURE-MEDIA AND THEIR PREPARATION 91 



entiate species, and to secure the organism in quantities sufficient 

 for preparation of vaccines, antitoxins, etc. A few standard media 

 are commonly used in the laboratory for the growth of bacteria, and 

 a great variety of special types have been devised for certain 

 species that do not grow upon these. It is impracticable even 

 to enumerate the many special media that have been employed. 



LIQUID MEDIA 



Bouillon or Beef Broth from Meat. This is the commonest of 

 laboratory media and serves as a basis for the preparation of 

 many others. 



Place 500 gm. chopped lean beef in a liter of water and allow 

 it to stand in a refrigerator over night. The juice is then pressed 

 out with a meat press, boiled for half an hour, the coagulated 

 albumins filtered out, the liquid made up to a liter with water, 10 

 gm. of peptone added, and heated sufficiently to dissolve. The 

 reaction is adjusted to the proper point, usually 4- 1, by titra- 

 tion, or the medium is simply neutralized by addition of nor- 

 mal XaOH, using phenolphthalein paper as an indicator if a high 

 degree of accuracy is not required. The broth is then autoclaved 

 at 15 pounds pressure or boiled for fifteen minutes, allowed to 

 cool, and then filtered. The cooling throws down a precipitate 

 of magnesium ammonium phosphate, which may then be removed. 

 In many cases this is not objectionable and filtration may be 

 carried out while the solution is still hot. The finished bouillon 

 or broth is placed in test-tubes and flasks, and sterilized in the 

 autoclave under a pressure of 15 pounds for 15 minutes. 



Bouillon or Broth from Beef Extract. It is customary, in much 

 of the routine work of the laboratory, to substitute for the pre- 

 ceding a broth in which three grams of a beef extract, such as 

 Liebig's, is substituted for the meat. 



Sugar-free Broth. There is generally present in the preceding 

 media a small amount of carbohydrate, largely dextrose. In 

 some cases a sugar-free medium is required. Theobald Smith has 

 devised a modification of the meat broth for this purpose which 

 is commonly used. Several broth tubes containing vigorous 

 twenty-four-hour cultures of Bacillus coli are added to the meat 

 infusion and kept at 37 for eighteen hours. In this time the 



