2/2 BACTERIOLOGICAL ANALYSIS OF MILK. 



I c.c. of A in 19 c.c. of .water gives a dilution of 2,000. 



y 2 c.c. of A in 191/2 c.c. of water gives a dilution of 4,000. 



i c.c. of A in 99 c.c. of water gives a dilution of 10,000. 



Higher dilutions of course can be obtained by simple calcu- 

 lation. 



Making Plate Cultures. Meantime a number of tubes of 

 gelatin (either peptone or whey gelatin, or both) are melted. 

 With a sterilized pipette two c.c. of the litmus solution are 

 adde(l to each tube of gelatin. A single c.c. of the final 

 dilution of the milk, after thorough shaking, is removed by a 

 clean sterilized pipette and placed in each of the test-tubes of 

 gelatin, and then the contents of each tube is gently but thor- 

 oughly shaken until completely mixed. There is thus ob- 

 tained a deep-blue mixture. The contents of the tubes are 

 to be poured out into petri dishes in the ordinary way. At 

 least three plates of each dilution chosen should be prepared, 

 making six plates in all, one half more highly diluted than 

 the other half. It is necessary to have the different dilutions, 

 since if the plates are too crowded with colonies the differen- 

 tiation which is to follo.w is not satisfactory. A little ex- 

 perience is required before one can judge of the proper dilu- 

 tion to make, but after a few experiments it is commonly easy 

 to fix upon the dilution which is likely to be satisfactory. 



For reasons to be presently mentioned it is desirable to 

 prepare one or two agar plates at the same time with the 

 litmus. It is done in the same ,way. Ordinary agar tubes 

 (preferably with 2% agar) are melted and to each is added 

 two c.c. of the litmus and one c.c. of the diluted milk. The 

 rest of the procedure is the same as with the gelatin cultures. 



Before these plates can be satisfactorily studied it is neces- 

 sary to allow them to grow for several days. The reason for 

 this is that not until about the sixth day are the different 

 colonies on the plates well enough developed to be readily 

 distinguished from each other. Sometimes, however, it is 

 impossible to allow the plates to grow for such a length of 



