CUL TI VA TION OF BA CTERIA . 1 99 



bonate may be necessary to give it a distinct bine. The 

 use of litmus is probably the best method of determining 

 whether bacteria by their growth produce acids or alka- 

 lies. 



The watery solution of litmus, being a vegetable in- 

 fusion, is likely to spoil; hence it should always be treated 

 like the culture-media and sterilized by steam every time 

 the receptacle in which it is kept is opened. 



Petruschky's Whey. — In order to differentiate be- 

 tween acid and alkaline producers among the bacteria, 

 Petruschky has recommended a neutral whey colored 

 with litmus. It is made as follows: 



To a liter of fresh skimmed milk 1 liter of water is 

 added. The mixture is violently shaken. About 10 c.cm. 

 are now taken out as a sample to determine how much 

 hydrochloric acid must be added to produce coagulation 

 of the milk, and, having determined the least quantity 

 required for the whole bulk, it is added. After coagulation 

 the whey is filtered off", exactly neutralized and boiled. 

 After boiling it is generally found clouded and acid in 

 reaction. It is therefore filtered again, and again neu- 

 tralized. Litmus is finally added to the neutral liquid, so 

 that it has a violet color, which can readily be changed to 

 blue or red by alkalies or acids. 



The medium is a very useful aid in differentiating 

 the typhoid and colon bacilli, showing well the alkaline 

 formation of the typhoid bacillus. 



Peptone Solution, or Dunham's solution, is very use- 

 ful for the detection of certain faint colors. It is a per- 

 fectly clear, colorless solution, made as follows: 



Sodium chlorid, 0.5^ Boil until the ingredients 



Witte's dried peptone, 1. V dissolve; then filter, fill 

 Water, 100. J into tubes, and sterilize. 



It is one of the best media for the detection of indol. 

 In it the bacillus pyocyaneus produces its blue color. A 

 very important fact in regard to peptone has been pointed 



