STANDARD MEDIA 77 



2. Sugar Broths. Broths to which sugars are to be added 

 are prepared in the same manner as nutrient broth, adding i 

 per cent, of dextrose, lactose, saccharose or other sugar. The 

 sugar is to be added before sterilizing. Sterilizing in the auto- 

 clave is to be preferred because the longer steam sterilization is 

 apt to cause inversion of the sugar. The reaction of the sugar 

 broths shall be neutral to phenolphthalein. 



These media are much used in testing for the presence of 

 Bacillus coli (dextrose broth). The committee states that the 

 removal of muscle sugar by inoculating with B. coli is not nec- 

 essary if small amounts of gas formation are to be disregarded. 

 In the routine work of testing water for the presence of the B. 

 coli a sufficient volume of the water to be tested is added so that 

 the resulting mixture will be one of normal strength. The com- 

 mittee also advises against the use of beef extracts in place of the 

 laboratory made beef infusions. 



3. Nutrient Gelatin. Make the beef infusion in the manner al- 

 ready described. After the first filtering through cotton or cotton 

 flannel, add 10 per cent, of gelatin (the per cent, being based on the 

 weight of the beef infusion instead of volume and the weight of 

 the gelatin to be on a basis of dry condition, and i per cent, of 

 peptone) and warm over water bath with constant stirring until 

 the peptone and gelatin are entirely dissolved. While dissolving 

 the peptone and gelatin the temperature should not rise above 

 60 C. Boil for 2 min. and adjust the reaction to +1.00 per 

 cent. Heat for 40 min. over water bath or in steam sterilizer 

 and restore any loss by evaporation. Again adjust the reaction 

 if necessary and boil over open flame for 5 min. with constant 

 stirring. Restore loss, filter until clear, titrate and record this 

 final reaction. Tube and sterilize as for beef broth and at once 

 store in ice chest. Protect against evaporation as already 

 explained. 



4. Nutrient Agar. Boil 15 grams (dry weight) of washed 

 thread agar in 500 cc. of distilled water for half an hour and make 



