PREPARATION OF CULTURE MEDIA. IQ 







AGAR is the name given to the dried strips of a 

 Japanese sea- weed. It forms a jelly which differs from 

 that containing gelatin in that it melts at a higher 

 temperature; nutrient agar as used in the laboratory 

 melts just below the boiling point of water and sets at 

 about 40 C. This is an advantage in the cultivation 

 of most pathogenic bacteria, for these grow (as a rule) 

 best at or near the temperature of the body, the 

 temperature to which they are exposed under natural 

 circumstances; and at this temperature gelatin would 

 melt. Agar is somewhat difficult to prepare unless the 

 practitioner has an autoclave, and may be bought with 

 advantage. But the following method is not very 

 difficult, and, as agar is perhaps the most generally 

 useful of all media, should be learnt. 



Requisites: i. Broth. 



2. Agar-agar. This should be cut up into very small 

 pieces with a pair of scissors, or may be bought in 

 powder. 



3. Solution of acetic acid. (Glacial acetic acid 2-4 

 c.c., water 500 c.c.). 



4. A large beaker. 



5. Other apparatus and materials as for gelatin. 

 Mctiwd. Weigh out 2 grammes of agar for each 100 



c.c. of broth to be used, and soak it in the dilute acetic 

 acid for a quarter of an hour. Now strain off the acid 

 and wash the agar in water until a small piece does not 

 redden blue litmus paper when pressed upon it. Place 

 the broth in a glass beaker and add the agar. Now 

 place the beaker upon a piece of wire gauze upon a 

 tripod stand, and apply a small Bunsen flame or spirit 

 lamp ; this must be placed so that the flame impinges 

 on a point not far from the side of the beaker. As the 

 fluid is heated it will rise and a continual circulation 



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