234 STATE BOARD OF AGRICULTURE 



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tion differs, in that milk requires more time, because of the very resistant 

 bacteria that may be present in it. The bouillon, it will be remembered, 

 had undergone several heatings before pouring it into tubes. The milk 

 on the other hand had been continuously exposed since it left the udder of 

 the cow. One half hour each day for three successive days is necessary 

 for its complete sterilization. Many other liquid media are made for the 

 cultivation of bacteria, but most of them have special objects; for this rea- 

 son they will not be given consideration. 

 Solid Media. Solid media have two distinct functions, that of yielding char- 

 acteristic growths and that of isolating species, both of which 

 will be reviewed later. To accomplish these purposes it is desirable that 

 these solidmedia be transparent and that they will liquify and solidify under 

 certain conditions. Gelatin and agar-agar satisfy the above 

 Gelatin. requirements. Gelatin is a substance obtained from animal 



tissues and is used extensively in household cookery. However 

 the gelatin thus used does not answer for bacteriological work which calls 

 for the best silver or gold leaf quality. When this substance is dissolved 

 in water it gives a jelly-like mass; when hot it is liquid and when cold 

 solid. Perfect transparency also exists. It will not be necessary to point 

 out the difficulties to be met in the making of gelatin medium nor state 

 with exactness the steps of the process; suffice it to say what constitutes 

 gelatin medium. It is simply the bouillon with ten or twelve per cent of 

 gelatin added and dissolved. When it has been made properly, it is trans- 

 parent, solid when cold, and of a suitable reaction. This gelatin will 

 liquify at 24° C. [75° F. ]. Sterilization is accomplished the same as 



bouillon, fifteen minutes each day for three successive days. 

 Agar-agar. ^ dried sea weed has been called agar- agar. When placed in 



water it swells and gradually dissolves. In solution it forms 

 a very hard jelly-like material. Without attempting to describe in detail 

 the process of making, agar-agar may be substituted for gelatin. It yields 

 a transparent medium which will liquify at 85° C. [193° F.] and solidify 

 at 42° C. [108° F.]. This medium is especially adapted to the cultivation 

 of bacteria needing an incubator temperature 37^° C. [98° F.], for which 

 purpose gelatin would not answer. One half hour each day for three suc- 

 cessive days is required in the sterilization of agar because of the slowness 



with which it liquifies. 

 Blood Besides gelatin and agar, blood serum is commonly used, 



erum. This is the clear fluid rising from clotted blood and contains 



large amounts of albuminous material. That its transparency 

 may be preserved, it is placed in tubes as the other media and sterilized at 

 60° C. [140° F.] for two hours each day for several days; when it is steri- 

 lized, it is subjected to a heat of 70° C. [166° F.] for the purpose of solid- 

 ifying. It is usually inclined in the tubes to increase the surface and 

 solidified at once. In this case the usual temperature could be employed 



to sterilize. 

 Potato. Qj^e other medium must be mentioned as a general cultural 



medium. It is the common Irish potato. This is boiled, cut 

 in cylinders the size of the tubes, then diagonally cut from top to bottom 

 in halves. By this operation a good surface is presented and each half is 

 placed in a tube. The tubes are then sterilized fifteen minutes each day 

 for three successive days. 



All of these media, both liquid and solid, contain the necessary constitu- 

 ents for the development of bacteria. Each finds its use under different 



