MEAT BROTH 159 



obtaining bacteria for the study of morphology and cell 

 groupings and for ascertaining many of the physiological 

 activities of the organisms. Solid media are useful for 

 studying some few of the physiological activities and espe- 

 cially for determining characteristic appearances of the 

 isolated growths of bacteria. These isolated growths of 

 bacteria on solid media are technically spoken of as ''colo- 

 nies,'' whether they are microscopic in size or visible to the 

 unaided eye. 



It is clear that the kinds of culture media used for the 

 study of bacteria may be unlimited but the undergraduate 

 student will need to use a relatively small number, which 

 will be discussed in this section. 



Meat Broth (Bouillon). i— This itself is used as a medium 

 and as the basis for the preparation of other solid and liquid 

 media. 



Finely ground lean beef is selected because it contains the 

 necessary food materials. Fat is not desired, since it is a 

 poor food for most bacteria and in the further processes of 

 preparation would be melted and form an undesirable film 

 on the surface of the medium. The meat is placed in a 

 suitable container and mixed with about twice its weight of 

 cold water (not distilled) and allowed to soak overnight or 

 longer. The cold water extracts from the meat water- 

 soluble proteins, blood, carbohydrates in the form of dex- 

 trose (occasionally some glycogen), nitrogenous extractives 

 and some of the mineral salts. The fluid is strained or 

 pressed free from the meat. This "meat juice" should now 

 be thoroughl}^ boiled, which results in a coagulation of a 

 large part of the proteins and a precipitation of some of the 

 mineral salts, particularly phosphates of calcium and mag- 

 nesium, both of which must be filtered off and the water 

 loss restored by adding the proper amount of distilled water. 

 The boiling is done at this point because the medium must 

 later be heated to sterilize it and it is best to get rid of the 

 coagulable proteins at once. The proteins thus thrown out 



1 The exact laboratory details for preparing various media are not given 

 in this chapter. It is the object to explain the choice of different materials 

 and the reasons for the various processes to which they are subjected. 



