THE PREPARATION OF CULTURE MEDIA 135 



been said, it is apparent that artificial culture media must, to a 

 certain extent, be adjusted to the peculiarities of individual bacteria. 

 In the cases of the more strictly parasitic microorganisms growth 

 can be obtained only by the most rigid observance of special require- 

 ments. For the large majority of pathogenic bacteria, however, 

 routine or standard media may be employed, which, while slightly 

 more favorable for one species than for another, are sufficiently 

 general in their composition to permit the growth of all but the 

 most fastidious varieties. 



The basis of many of our common media is formed by the soluble 

 constituents of meat. These substances are best obtained by 

 macerating 500 grams of lean beef in 1,000 c.c. of distilled water. 

 The mixture is 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 manner given in the detailed in- 

 structions 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-extract media are less favor- 

 able for the cultivation 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 

 path'o genie microorganisms and hold an important place in labora- 

 tory technique. 



The ingredients and methods used in various laboratories in the 

 preparation of such standard media should be, as much as possible, 

 uniform, in order that confusion in results 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 composition. 



A committee of the American Public Health Association, 1 ap- 

 pointed in 1897 for the sake of standardizing the methods of prepara- 

 tion of media, recommended that the following rules should govern 

 the choice of ingredients : 



1. Distilled water should be used in all cases. (This is not neces- 



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

 Philadelphia, Sept., 1897, 



