22 EXPERIMENTAL DAIRY BACTERIOLOGY 



water blank by means of a sterile 1-cc. pipette, the liquid well 

 mixed, and 1 cc. of the mixture taken. To obtain 1/100 cc. 

 of milk, 1 cc. is placed in 99 cc. of water and 1 cc. taken ; 

 1/1000 cc. of milk is obtained by carrying 1/100 cc. to a 9-cc. 

 water blank and using 1 cc. of the mixture. Still higher dilu- 

 tions are obtained in a similar manner. In each plate is placed 

 the same amount of the mixture of milk and water, 1 cc, ; 

 thus the extent to which the medium is diluted is constant. 

 A clean pipette must be employed for each dilution. 



Media. A variety of media are used for the quantitative 

 analysis of milk. Each has certain advantages and disadvan- 

 tages. Gelatin melts at temperatures above 25 C., and is 

 liquefied by certain kinds of bacteria. If these forms are 

 numerous, the entire mass of gelatin will be rapidly lique- 

 fied and no definite results obtained. Agar melts at 96- 

 99 C. and does not solidify again until it has cooled to 

 39-40 C. It is not liquefied by any of the ordinary bac- 

 teria. Gelatin can therefore be used only at room tempera- 

 ture, while agar cultures may be incubated at blood heat. 

 Lactose agar is preferable for quantitative analysis of milk. 



Cooling. After the liquid medium has been poured into the 

 plates they must be tilted from side to side in order to dis- 

 tribute the medium evenly over the entire inner floor of the 

 dish. This must be done before the medium begins to solidify, 

 so that the appearance of the plate shall not be marred by a 

 rough surface. This is easily accomplished with gelatin, as 

 it cools slowly. With agar the medium must be allowed 

 to cool to 43-45 C. before it is inoculated, and since it 

 solidifies at 39-40 C., it requires more careful manipula- 

 tion to properly prepare cultures with this medium. More 

 uniform distribution can be had with agar if before adding 

 the medium the plates are slightly warmed by holding them 



