CULTURAL CHARACTERS OF BACTERIA 29 



10 minutes longer, when the medium should be perfectly clear. 

 Restore to original volume. Titrate with phenolphthalein ; 

 adjust reaction to final point desired. Boil for 5 minutes; 

 restore to original volume. Filter, tube, and sterilize. In filter- 

 ing agar or gelatin make a folded filter, and wet previous to 

 filtration with distilled water. If the latter media are perfectly 

 limpid they will rapidly run through the paper, and a hot water 

 funnel is unnecessary. Often the latter portion of the agar will 

 run through slowly. This can be hastened by placing the entire 

 filtering apparatus in an autoclave with some pounds' steam 

 pressure. In this case the funnel should be covered with a 

 glass plate, to prevent condensed steam being added to the 



medium. 



Milk 



It is absolutely important where milk is used as a culture 

 medium that it should be perfectly fresh. In stale milk 

 certain fermentative changes have already taken place which 

 render it unsuitable. It should be as free from fat as possible 

 by running it through a centrifugal separator. The resulting 

 skimmed milk is then immediately tubed and sterilized. Milk 

 is best sterilized at ioo C. for several days, as higher tempera- 

 tures are liable to discolor the medium. If the milk be acid in 

 reaction, it should first be made exactly neutral to litmus paper 

 before it is placed in the tubes, using the necessary quantity of 



normal NaOH. 



Litmus Milk 



Litmus milk is prepared from plain milk after the latter 

 is sterilized, by adding sufficient sterilized litmus solution to 

 give the medium a pale blue color. It is then distributed 

 under aseptic conditions to sterile tubes. The best kind of 

 vessel for holding the litmus milk is the well-known Lister 

 flask, with a side tube placed just below the neck from 

 which the medium can be poured with slight risk of con- 

 tamination. 



