THE MEDIA OF THE CULTIVATION OF BACTERIA. 59 



Not only does it serve as an excellent medium for the culti- 

 vation of nearly all bacteria that grow out of the incubator 

 temperature (36 to 37 C.), but it offers by the plate -method 

 one of the best culture-soils for isolating bacteria. 



As some of the bacteria liquefy gelatin and others do not, 

 it is useful also in the differentiation of these species. On 

 account of its clearness, its easy preparation, and the other 

 advantages mentioned above, it is one of the best culture- 

 soils available. 



The disadvantages lie chiefly in the fact that it liquefies at 

 a much lower temperature than that of the incubator, and 

 can not therefore be used for the cultivation of some patho- 

 genic bacteria which grow only at blood temperature. 



2. Agar. 



Nutrient agar is prepared much in the same manner as 

 gelatin, except that 20 or 30 grams of 2 or 3 per cent, agar 

 are added, instead of the gelatin, to the meat-infusion, and 

 the process of cooking must be much more prolonged (four 

 to five hours), as the agar is much slower in dissolving. It 

 is necessary also sometimes, after dissolving the agar and 

 neutralizing the mixture, to add the white of one or two eggs 

 in order to clarify the solution before filtration. This is done 

 by removing the flask from the fire and allowing it to cool to 

 a temperature below 70 C. After clarification the mixture 

 is again cooked for one and one-half to two hours and filtered 

 through absorbent cotton two or three times. This filtration 

 is a much more difficult process than with gelatin, and must 

 be carried on in the steam sterilizer. After filtration the 

 agar is distributed among test-tubes for use, and is sterilized 

 by the same procedure as nutrient gelatin. 



Nutrient agar has a melting-point much higher than that 

 of gelatin, about 42 C., so that it may be used for the culti- 

 vation of those bacteria which grow only at or best at the 

 temperature of the human body. It possesses all the advan- 

 tages of gelatin, but is not so clear and transparent, and is not 

 liquefied by the secretions of any known bacteria. It is useful 

 also for the isolation of bacteria by means of plate cultures. 



