BACTERIOLOGICAL ANALYSES 663 



Plating. 'For plating the following media are recommended : 

 Media for Total Counts and also for Acidifiers 

 4 grams beef extract 

 10 grams peptone 

 30 grams lactose 

 4 grams sodium chloride 

 12 grams thread agar 

 1000 cc. distilled water 

 Acidity 0.1 per cent. 



For acidifiers add 1 cc. of sterile litmus solution to each plate, 

 before pouring the agar. 



Media for Liquefiers 



4 grams beef extract 

 10 grams peptone 

 30 grams lactose 

 4 grams sodium chloride 

 150 grams gelatin 

 1000 cc. 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. Cv . 



Add 1 cc. of sterile 1 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 C. (95 F.) for at least three days before making counts. 

 Incubate gelatin plates at 21 C. (70 F.) for four to five days be- 

 fore 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. 



