82 Microbes and You 



II. Derived Media— Comprised of known substances but the exact 

 chemical composition of which is not known. 



1. Nutrient broth. 



2. Nutrient agar. 



3. Nutrient gelatin. 



III. Synthetic Media— The exact chemical composition is known. 



IV. Special Media— Combinations of the other three types of 

 media. 



PREPARATION OF STANDARD NUTRIENT AGAR 



There are literally hundreds of different combinations of in- 

 gredients comprising the workable list of media employed by micro- 

 biologists. Trying to remember the formula for very many media 

 is generally a waste of a person's time. But it doesn't seem too 

 much to ask students, even in an elementary course, to remember 

 the constituents of the most commonly used medium, Standard 

 Nutrient Agar. During a single semester each student may use a 

 hundred or more tubes of media. In many bacteriology courses 

 one of the early laboratory exercises is devoted to having students 

 prepare a batch of standard agar. Such an exercise should ac- 

 complish two objectives. First, the very fact that the operation 

 involves "doing" will help the student to remember the constituents 

 and the technic for blending them much better than merely hearing 

 the steps presented in a lecture. But even more important, per- 

 haps, the student should be impressed with the amount of time, 

 effort, and expense involved in the preparation of even one tube of 

 culture medium. 



STANDARD NUTRIENT AGAR 



*A firmer medium can be obtained by adding a higher concentration of agar 

 -usually up to two per cent. 



