116 



BIOLOGY AND TECHNIQUE 



m 





allowed to infuse in the ice chest over night, and then strained through 

 cheese-cloth. To this infusion are added the other required constituents 

 in the naanner given in the detailed instructions below. The soluble 

 constituents of meat, however, may also be procured in a simpler way 



by the use of the commercial meat extracts, 

 such as that of Liebig. These extracts are 

 dissolved in quantities of five grams to the 

 liter, and other constituents are added to 

 this nutrient basis. 



Though simpler to make, the meat-ex- 

 tract media are less favorable for the culti- 

 vation of the more delicate organisms than 

 are the media made directly from fresh meat. 

 Nevertheless, they suffice for the cultivation 

 of the large majority of the more saprophytic 

 pathogenic microorganisms and hold an im- 

 portant place in laboratory technique. 



The ingredients and methods used in va- 

 rious laboratories in the preparation of such 

 standard media should be, as much as pos- 

 sible, uniform, in order that confusion in re- 

 sults may be avoided ; for, as is well known, 

 the biological characteristics of one and the 

 same bacterial species may vary considerably 

 if grown on media differing in their com.po- 

 sition. 



A committee of the American Public 

 Health Association,' appointed in 1897 for 

 the sake of standardizing the methods of preparation of media, recom- 

 mended that the following rules should govern the choice of ingredients: 



1. Distilled water should be used in all cases. 



2. The meat used should be fresh, lean beef (when veal or chicken 

 is substituted the change should be stated). 



3. The pepton used should be Witte's pepton, dry, made from meat. 



4. Only C. P. NaCl should be used. 



5. For alkalinizing C. P. sodium hydrate should be used in normal 

 solutions. 



a b 



Fig. IS.^Test Tube (a) 

 incorrectly stoppered; (6) 

 correctly stoppered. 



' Rep. Com. of Amer. Bact. to Com. of Amer. Pub. Health Assn. Meeting, 

 Philadelphia, Sept., 1897, 



