266 CLEAN MILK 



(b) Plain nutrient agar: Add to " i to i meat infusion -f- 

 peptone " (i. e., 1,000 grams or c.c.) an equal quantity of "agar 

 jelly" (i. e., 1,000 grams), hot or cold, prepared by dissolving 

 thread agar (30 grams) in water (to make 1,000 c.c.) by boiling to 

 make a 3% solution (for milk work, 2%), which jellies on cooling. 

 For glycerine agar, plain dextrose, etc., agar, add constituents as 

 under broth. (Note: Plain nutrient agar, made as here directed, 

 except that the percentage of agar in the finished medium is 1% 

 instead of iy 2 %, and the reaction 1.5% acid instead of 1% acid, is. 

 the official count medium recommended by the Committee on 

 Standard Methods of Bacterial Milk Analysis.) 



(c) Plain nutrient gelatin: Add to " 1 to 1 meat infusion -f- 

 peptone " (i. e., 1,000 grams or c.c.) an equal quantity of " gelatin 

 jelly," hot or cold (prepared by dissolving sheet gelatin 200 grams 

 in water (to make 1,000 c.c), heated not above 40" C, to make a 

 20% solution, which jellies on cooling. For glycerine gelatin and 

 plain dextrose, etc., gelatin, add constituents as under broth. 



(d) For all media which should be muscle-sugar-free (either 

 for use as strictly sugar-free media, i. e., as indol broth, or to get 

 rid of all muscle sugar first, in order to add exact quantities of other 

 sugars, adjust the " 1 to 1 meat infusion+peptone " to the 1.5% 



(normal) acid point, as indicated below, warm to 37 C. and in- 

 oculate with a vigorous 24 hours culture of B. coli in plain broth. 

 Set in incubator at 37 C. for 12 hours (i. e., over night). In this 

 time all the sugar should be disintegrated by the bacillus. Restore 

 any loss by evaporation. Then proceed as under plain nutrient 

 broth, etc., according to the medium desired. (Note: Litmus solu- 

 tion, as described below, or azolitmin,* 2% solution, 1% of the total 

 mixture, may be added to any medium, as desired, at Step 5.) 



Note : A test for the presence of sugar may be made as follows : 

 —Fill a fermentation tube with the medium to be tested (after 



f This may be bought of makers of biological products. 



