PREPARATION OF MEDIA 41 



taining most media at 121°C for 15 min will not cause important changes. 

 The chemical composition of many substances, particularly carbo- 

 hydrates, is changed, however, even by this limited heat treatment (Davis 

 and Rogers, 1939 j. For critical study, such substances should be steril- 

 ized separately by filtration through bacteriological filters (Seitz or 

 sintered glass) and added aseptically to the remainder of the medium 

 previously sterilized by heat. If filtration facilities are lacking, concen- 

 trated stock solutions (20-25 per cent) of such substances can be sterilized 

 by heat in the autoclave and then dispensed aseptically to the desired 

 final concentration. In cases where conclusions based on comparative 

 studies of the sterilization modifications described above are not avail- 

 able, the effect of heat upon the medium must be considered in each new 

 investigation. 



Dehydrated media refer to powdered, water-soluble commercial 

 products which yield a growth medium. Usually all that is required in 

 the preparation is to dissolve the proper amount of the powder (according 

 to the directions accompanying the medium), to dispense as desired, and 

 to sterilize. A wide variety of types, each suitable for a specific purpose, 

 is available, and these have been found to be adequate for bacteriological 

 use. Indeed, in most instances, the greater uniformity of these products 

 over that attained by preparation of individual batches prepared in the 

 laboratory from separate ingredients indicates their desirability for com- 

 parative work. No attempt will be made here to describe each of these 

 dehydrated media (in some instances formulas for preparation of media 

 will be given even though satisfactory dehydrated products are avail- 

 able), but those persons desiring to use these products should seek 

 information from the manufacturing companies. Companies in this 

 country which specialize in dehydrated media preparation include 



(1) Albimi Laboratories, Inc., 16 Clinton Street, Brooklyn 1, New York, 



(2) Baltimore Biological Laboratories, 1640 Gorsuch Avenue, Baltimore 

 18, Maryland, and (3) Difco Laboratories, Inc., 920 Henry Street, 

 Detroit 1, Michigan. Products such as peptone, beef or yeast extract, 

 agar, etc., are also available from other supply houses not specializing in 

 dehydrated media preparation. 



CULTIVATION AND STORAGE MEDIA 



The media to be described in this section will include the formulas for 

 various complex, nonsynthetic media which may be used for the general 

 cultivation of bacteria, either from a natural sample or after a pure cul- 

 ture has been obtained. No attempt will be made to designate any one 

 medium as the standard for a particular purpose, but it may be noted 

 that for certain purposes (for example, estimation of organisms present 



