EXPERIMENT STATION BULLETINS 233 



PREPARATION OF MEDIA. 



Up to this point the requirements of bacteria have been given due con- 

 sideration. The nature of their food has been discussed, and the elements 

 constituting that food; it has also been stated that moisture is an essential 

 feature of the food, and that a suitable reaction is necessary. These are 

 the factors which must enter into the nourishment of bacteria. With 

 these data it becomes possible for the laboratory worker to prepare a 

 medium which will embrace these conditions; but besides the mere matter 

 of a proper nourishment, the experimenter has learned that different 

 species of bacteria produce different growths upon the various media or 

 foodstuffs, and this fact he utilizes in identifying bacteria; consequently, 

 certain media have an advantage over other media. The investigator, 

 therefore, strives to provide a medium which will not only answer the 



requirements of food but also those of a biological character. 

 Classes. j^ jg customary to divide media into two distinct classes, 



liquid and solid media. Each has its particular purpose in the 



laboratory. For the inoculation of animals, the study of bac- 

 Liquid terial products, and mere cultivation, liquid media aresatisfac- 



Media. tory. When animals are inoculated with bacteria, a syringe is 



employed which requires liquid media. In the study of bac- 

 terial products, a liquid substance is treated more easily. To transplant 

 bacteria with the object of keeping them alive, liquid media, which is 

 readily made, is suitable. While cultural properties are not so pronounced 

 in liquid media, they however have their significance, and in some of these 

 media as, for instance, milk, this feature is very valuable. 



There are many liquid solutions which are especially adapted to the 



growth of bacteria. Milk has been mentioned. More generally 

 Bouillon. used than milk and better fitted for ordinary work is bouillon. 



This is owing to its perfect clearness and freedom from sedi- 

 ment; at the same time it possesses nutrient material of the widest range. 

 To make it, one pound of chopped lean meat is shaken in one quart of 

 water and allowed to stand for twenty-four hours or steeped for one hour 

 that the meat extracts and soluble salts may pass into solution. The insol- 

 uble albuminous portion of the meat is strained off and to the filtrate is 

 added one per cent of peptones which are soluble and not coagulable by 

 heat. These take the place of the meat that was strained off. One half 

 per cent of ordinary salt is added to facilitate the solution of the peptones 

 When everything is dissolved a suitable reaction is obtained by the addition 

 of some alkali, for the meat solution is usually acid. At this stage the 

 mixture is heated, and boiled for one hour. It should then be clear and the 

 sediment formed may be filtered off. It is now ready to pour into tubes of 

 glass, called test tubes, which have been plugged with coi ton-wool and 

 sterilized in the hot air oven for one hour at 150° C. [302° F.]. Having 

 filled these tubes to the depth of one and one-half inch with the bouillon, 

 they with their contents are sterilized in steam heat for fifteen minutes 

 each day for three successive days, as directed under sterilization. At the 

 end of the third sterilization, the bouillon tubes are free from bacteria and 



are ready for the reception of any species. 

 Milk. Milk tubes are prepared much as bouillon. The milk is usu- 



ally employed unmodified and poured into tubes which have 

 been prepared in the same way as those used for bouillon. The steriliza- 



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