274 CLEAN MILK 



liquefaction further assists in recognition. Plates, to remain 

 solid, must be incubated at about 22° C. The great objections 

 to gelatin for plates are the difficulty of maintaining the tem- 

 perature at 22 C, and the liquefaction due to some species, 

 which usually quickly destroys the chief value of a plating 

 medium — -its solidity. 



'Modifications of agar and gelatin media — the sugars are added 

 chiefly to elicit gas formation (best tested in stabs) ; litmus 

 to indicate changes in reaction; glycerine, because certain 

 bacteria are believed to grow better in its presence ; milk for a 

 similar reason. 



Plain nutrient broth — used chiefly for characteristics of growth on 

 liquids and to elicit the typical arrangements of cells when 

 growing freely (i. e., chains, clusters, etc.) ; also for motility, 

 which develops better in liquids than on solids. Tube 5 c.c. in a 

 test tube; in fermentation tubes, use enough to half fill bulb 

 and completely fill closed arm. The modifications (with 

 sugars, litmus, glycerine, etc.) are made for the reasons given 

 under agar and gelatin. Gas formation is best determined 

 quantitatively and qualitatively, by the use of sugar broths in 

 fermentation tubes. 



Potato — for gross characteristics of growth. 



Milk — for the detection of coagulation of milk, the subsequent so- 

 lution of the clot, and other evidences of digestive action. 

 Milk is too full of casein particles, fat and albumin to yield 

 good smear preparations for morphological study as a rule. 

 Combined with litmus, acid or alkali formation can be de- 

 termined, and changes in reaction noted. Litmus milk often 

 becomes decolorized by bacterial absorption of the oxygen 

 necessary to its coloration. 



