86 MICROBIOLOGY 



Serum may be sterilized, when great care is used, by 

 passing it through a well-tested Pasteur filter, under pres- 

 sure. When so treated the filtrate is very clear and light 

 colored. 



Preparation of milk for a culture medium. It is better 

 that the cream be removed from the milk before it is used. 

 To do this the fresh milk is placed in a beaker and set in the 

 ice box for from 10 to 15 hours. Then carefully remove the 

 cream. Fresh separated milk can be used with advantage. 

 It is well to filter the milk through a thin layer of absorbent 

 cotton to remove any masses of cream that may be left. The 

 reaction should be tested, and if strongly acid, should be 

 rejected or made 1.5% acid to phenolphthalein by the addi- 

 tion of N/10 sodium hydrate. Distribute the skimmed milk 

 in small test tubes (5 cc. in each) and sterilize by discontinu- 

 ous steaming. Wipe, label, and store. 



Preparation of litmus milk for a culture medium. This 

 is prepared the same as the milk medium, with the addition 

 of enough of an aqueous solution of litmus to impart a decid- 

 edly blue color to the milk. Sterilize, wipe, label, and store 

 the same as the milk. 



Litmus media. When it is desirable to determine 

 whether bacteria produce in their growth acid or alkali from 

 one or more of the constituents of the media litmus is fre- 

 quently added. To prepare the litmus solution take the 

 lump litmus, powder finely, and boil with distilled water so 

 that a saturated solution is obtained. Filter and then boil for 

 thirty minutes on two successive days. The litmus solution 

 is added to the neutral media in sufficient quantity to give 

 the desired depth of color. The less heating that is done 

 after mixing the better the results. Merck's purified litmus 

 in 1 per cent aqueous solution should be used in careful work. 



Petrusky's litmus whey (as modified by Durham). Fresh 

 milk is slightly warmed and clotted by means of essence of 

 rennet. The whey is strained off and the clot is hung up to 

 drain in a piece of muslin. The whey, which is somewhat tur- 

 bid, is then cautiously neutralized with 4 per cent citric acid 



