CULTURAL METHODS 79 



related to proteins. Usually from ten to fifteen per cent 

 of gelatin is required in order to cause a medium to solidify 

 when cool. It is not adapted to the solidification of media 

 used for the growth of organisms requiring temperatures 

 higher than average room temperature. Bacteria, for ex- 

 ample, which grow only at the temperature of the body 

 cannot well be tested on gelatin as this medium would be 

 usually liquid at this temperature. 



Agar-agar is a complex carbohydrate prepared from cer- 

 tain kinds of seaweed. Its solutions resemble those of 

 gelatin in their power of gelatinizing when cool. From 

 one to three per cent is ordinarily used in preparing solid 

 media. It may be noted that the melting temperature and 

 the temperature at which the agar medium solidifies are not 

 identical. A tube of bouillon agar, for example, must be 

 brought nearly to the boiling point before the agar liquefies, 

 but may be cooled to a temperature of forty-two to forty- 

 five degrees before it begins to solidify. The agar-agar is 

 not attacked by microorganisms generally, two or three 

 species only are known which can dissolve or digest it. 

 Gelatin, on the other hand, is readily attacked by many 

 species of bacteria, and one of the differential characters 

 noted is the ability of certain forms to digest or liquefy the 

 gelatin. 



By the addition of agar or gelatin practically any of the 

 ordinary liquid media may be solidified. 



Many other types of media and variations in the media 

 thus far mentioned will be noted later in the discussion of 

 the growth requirements of the specific kinds of bacteria. 



ADJUSTMENT OF THE REACTION OP THE MEDIUM 



The reaction of a medium, that is, its relative acidity or 

 alkalinity, may be designated in one of two ways: first, by 

 the amount of normal acid or normal alkali required to 

 bring one hundred cubic centimeters of the medium to the 



