376 BACTERIOLOGICAL ANALYSES 



30 grams lactose 

 4 grams sodium chloride 

 150 grams gelatin 

 1000 c.c. distilled water. 



Acidity 0.1 per cent. 

 Media for Yeasts and Molds 

 4 grams beef extract 

 10 grams peptone 

 12 grams agar 

 1000 grams whey 



Acidity 0.2 per cent. 



Add 1 c.c. of sterile one per cent tartaric acid solution to 

 each plate before pouring the medium over the dilution. 



Incubation. Incubate agar, litmus agar and whey agar 

 plates at 35 degrees C. (95 degrees F.) for at least three days 

 before making counts. Incubate gelatin plates at 21 degrees C. 

 (70 degrees F.) for four to five days before making counts. 



Making Counts. The colonies on the plates are counted 

 most conveniently by placing the plates over a standard counting 

 plate. In the absence of such a plate, place the petri dish upside 

 down on a dark surface and draw, with a blue crayon, radial lines, 

 dividing the field into segments. For plates containing not to 

 exceed 100 colonies eight to sixteen segments are sufficient for 

 easy counting. 



Qualitative Determinations. Numerical counts on the four 

 kinds of media recommended above usually furnish a fair general 

 idea of the types of bacteria present. 



For the detection of gas-producing species, nutrient bouillon 

 containing three per cent lactose and three per cent sucrose, re- 

 spectively, in fermentation tubes, or nutrient agar containing 

 three per cent lactose and three per cent sucrose, respectively, 

 in test tubes, are serviceable. 



Cans of sweetened condensed milk that show gaseous fermen- 

 tation (swell heads) are usually due to certain species of yeast, 

 which thrive best in media containing sucrose. 



Cans of evaporated" miilk that show gaseous fermentation 

 (swell heads) are usually caused by anaerobic putrefactive bac- 



