CULTURE MEDIA 3 



These considerations can help one to reaUze why such a great 

 variety of culture media have been proposed at various times and 

 are used in microbiological studies. These media are frequently 

 modified as regards the concentration of certain of the nutrients, 

 reaction, buffer content, elimination of one nutrient and sub- 

 stitution of another. 



GENERAL DIRECTIONS FOR THE PREPARATION OF CULTURE 



MEDIA 



Of the numerous formulae of various culture media, which have 

 frequently been only briefly described, only the most essential 

 and those which have been tried repeatedly and found useful 

 are given in this manual. It is assumed that the reader has at 

 hand a manual of general bacteriology, such as the Manual of 

 Methods Prepared by the Society of American Bacteriologists; 

 hence, all directions for the preparation of standards, for measur- 

 ing the hydrogen-ion concentration, buffer content, etc. are 

 omitted. The formulae are arranged according to the general 

 physiological characters of the microorganisms. 



For general purposes, the reaction of culture media for bacteria 

 should be about the neutral point, or pH 7.0. Some will grow 

 at a considerably lower pH value, others will grow only at the 

 neutral point or even at a more alkaline reaction. In some 

 cases, bacteria can be separated from one another, by merely 

 adjusting the reaction to such a point as to eliminate one group 

 of organisms without injuring the other. Fungi are able, as a 

 rule, to grow at a much higher acidity than bacteria. This fact 

 is utilized frequently for the separation of these two groups of 

 organisms: by adjusting the reaction of the medium to pH 4.0, 

 the bacteria are practically eliminated, while the great majority 

 of fungi are not affected. 



When it is necessary to adjust the reaction of a medium, the 

 scheme outlined here will be found convenient. Place 2 cubic 

 centimeters of the medium and 8 cubic centimeters of water in 

 a test tube and add 4 or 5 drops of phenol red or any other of the 

 desired indicators. Now add 0.1 iV or 0.05 N sodium hydroxide 

 from a burette until the color of the solution matches that of a 

 known standard. Calculate and add to the medium the amount 



