MTRIENT AGAR. 21 



PEPTONE SOLUTION (DUNHAM'S). 



Dissolve i' , of Witte's peptone and 1/2% of sodium chloride in 

 distilled water. Filter, tube and sterilize. Peptone solution may he 

 used as a base for sugar media instead of bouillon. It is the medium 

 used in testing for indol production. This test is made by adding 

 from six to eight drops of concentrated H 2 SO 4 to a 24- to 48-hour-old 

 peptone culture of the organism to be tested. If the organism pro- 

 duces both indol and a nitroso body, we obtain a violet-pink coloration, 

 "cholera red." If no pink color is produced on the addition of the 

 sulphuric acid, add about i c.c. of an exceedingly dilute solution 

 (i : 10,000) of sodium nitrite. 



XITRIEXT AGAR. 



In making agar medium it is preferable to use powdered agar, as 

 this goes into solution more readily than the shredded agar. The 

 reaction of agar is slightly alkaline, so that if i 1/2 to 2% of agar is 

 added to nutrient bouillon having a reaction of + 1 the finished prod- 

 uct will be found to be about -f .8. 



To make: Weigh 15 to 20 grams of powdered agar and place in a 

 mortar. Make a paste by adding nutrient bouillon, little by little, 

 and when a smooth even mixture is made, pour it into the inner com- 

 partment of a rice cooker and add the remainder of the 1000 c.c. of 

 bouillon. The use of the balance is preferable. 



The outer compartment of the rice cooker should contain the 25% 

 salt solution. Bring to boil, and the agar will be found to have 

 entirely gone into solution after three to five minutes of boiling. 



Then, using a funnel which has been heated in boiling water and 

 which contains a small pledget of absorbent cotton, we filter the agar, 

 tube it and sterilize it in the autoclave or Arnold. One and one-half 

 percent agar can be readily filtered through filter-paper and gives a 

 clearer medium. 



By taking of meat extract 3 grams, peptone 10 grams, salt 5 grams, 

 powdered agar 15 grams, the white of one egg and 1000 c.c. of water, 

 making at first a paste of all the ingredients in a mortar, then gradually 

 adding the remainder of the 1000 c.c. of water, putting in the rice 



