136 PATHOGENIC BACTERIA. 



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 i 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, i. 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 

 out by Garini, 1 who found that many of the peptones 

 upon the market were impure, and on this account failed 

 t<> show the indol reaction for bacteria known to produce 

 indol. He recommends the use of the biuret reaction 

 for testing the peptone to be employed. The reagent 

 used is Fehling's copper solution, with which pure pep- 

 tone strikes a violet color not destroyed upon boiling, 



1 Ctntralbl.f. Bakt. u. Parasitcnk., 1893, xiii., p. 790. 



