folding over one of its comers, and, applying the smooth end of a glass 

 rod to the folded portion, gently pushing it into the mouth of the tube. 

 After plugging and before media are introduced into the tubes and 

 flasks, these should be sterilized. This is best done in one of the " hot- 

 air sterilizers " (see Fig. 8, p. 69) , by exposing the tubes for one hour 

 to a temperature of 150 C. If greater speed is desired exposure to 180 

 to 190 Cj for half an hour is usually safe. If by mistake, however, the 

 temperature is allowed to rise above 200 C., a browning of the cotton 

 plugs occurs and the glassware is apt to be stained by the burning of 

 the fat and other organic material derived from the cotton. Petri dishes 



FIG. 17.-^PETRi DISH. 



after cleansing are fitted together in the manner shown in Fig. 17, 

 and are sterilized in the hot-air chan^r at 150 C. for one hour. 



Glassware so prepared is ready for the reception of media. 



Ingredients of Culture Media. The food requirements of bacteria 

 have been discussed in another section. From what has there 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 requirements. 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 per- 

 mit 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 



