MEAT BROTH 159 



solid media are technically spoken of as "colonies," 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).^ — This is used as a medium itself, 

 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 dextrose 

 (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 thor- 

 oughly 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 magnesium, 

 both of which must be filtered off and the water loss restored 

 by adding the proper amount of distilled water. The boil- 

 ing is dojie 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 

 deprive the medium of valuable nitrogenous food material 

 which is replaced by adding about 1 per cent, by weight of 

 commercial peptone. It is usual also (though not always 

 necessary) to add about 0.5 per cent, by weight of 

 common salt which helps to restore the proper concentra- 

 tion of mineral ingredients lost by the boiling. The chlorine 

 is also an essential element. The reaction is now deter- 



• 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. 



