PREPARATION OF MEDIA 81 



teria will not grow if the media are too acid or too alkaline. 

 Ordinarily a reaction -j-1.5 to phenolphthalein * is suitable 

 for the pathogenic species except perhaps Bact. mallei. The 

 media must be sterile and when possible it is desirable to have 

 them clear. 



As the species of bacteria are determined largely from 

 their cultural characters on the different media, it is neces- 

 sary to have a considerable variety of media in order to pro- 

 cure a range for their biochemic and physiologic properties. 

 Some of these have been designated as differential media, that 

 is, by their use certain species may be identified. Some species 

 of bacteria require for their multiplication certain kinds of 

 media such as blood serum, or glycerinated agar. These are 

 recognized as special media. 



The more commonly used media have for their base the 

 aqueous extract of meat, usually beef or veal, and peptone. 

 In addition to these, certain vegetables such as potatoes are 

 often used as well as such substances as milk, eggs, blood 

 serum and ascitic fluid. A large variety of media have been 

 used for the determination of certain properties of bacteria. 



LIQUID MEDIA. 



The more common of these are the different bouillons 

 and milk. Bouillon is the liquid medium most commonly em- 



* Titraticn of bouillon. Take 5 cc. of bouillon and place in a 

 porcelain evaporating dish with about 45 cc. of water. Boil for 

 three minutes. Add 1 cc. of a solution of phenolphthalein. Stir 

 and add to the solution in the evaporating dish enough of a N/20 

 NaOH solution from a burette to give it a clear, bright pink color. 

 This amount then of N/20 NaOH solution is required to neutralize 

 5 cc. of the bouillon. A provisional standard reaction of + 1.5 to 

 phenolphthalein has been adopted. This would be slightly alkaline 

 to litmus. In order to bring the entire amount of bouillon to the 

 desired reaction (+1.5), subtract 1.5 from the amount of N/20 

 NaOH solution drawn from the burette and multiply the difference 

 by the number of cc. of bouillon divided by 100. The product, if 

 plus, represents the amount of N/l NaOH to be added; if minus, the 

 amount of N/l HCL to be added. After mixing, test the reaction 

 again in the same way and add alkali or acid if needed. For greater 

 accuracy further tests should be made. 



