94 SOIL BACTERIOLOGY 



Medium 5 



Casein Agar 



Agar 10 gm. 



Casein 10 gm. 



Sodium hydroxid N/i (NaOH) 7 c.c. 



Distilled water 1000 c.c. 



1. Measure into an Erlenmeyer flask 100 c,c. of distilled 

 water and 10 grains of casein (Hammarsten). 



2. Add to this 7 c.c. of normal NaOH. 



3. Heat in a double boiler or steamer to get a perfect 

 solution. 



4. Dissolve 10 grams of agar in 900 c.c. of distilled water. 



5. Mix and filter the casein-agar solution. 



6. Adjust the reaction between (+0.1) and (+0.2) per 

 cent. If the casein is weighed accurately and the normal 

 solution is correct, the reaction will be about (+0.2). 



7. Tube, and sterilize in autoclave for fifteen minutes. 

 Cool quickly in cold or iced water. 



Note. — The final reaction of the medium will be about (+0.1), Fuller's 

 scale. If the medium is alkaline, the bacterial growth will be restricted. 

 If the medium is more than (+0.1) some of the casein may be precipitated 

 during steriHzation. The casein agar should be clear and almost colorless 

 when poured into a Petri dish. Sometimes the casein will be slightly pre- 

 cipitated during the sterilization or the cooling. This is of no consequence, 

 since the precipitate, when poured into plates, is so finely divided that it 

 becomes invisible. Casein agar should be incubated for six days at 30*^ C. 



Ayres, S. H., United States Dept. Agr. Bur. Animal Indus., 28th Ann. 

 Report, pp. 225-235, 1911. 



Medium 6 



Soil-extract Agar 



Agar 15 gm. 



Dextrose (CeHisOe) i gm. 



Soil extract 100 c.c. 



Water 900 c.c. 



