240 Cultivation of Micro-organisms 



If litmus be added to the milk before sterilization, it is apt 

 to be browned or decolorized, so that it is better to sterilize 

 the two separately and pour them together subsequently. 

 It is said that lacmoid is never thus changed, and many 

 workers prefer it to litmus on that account. 



Petruschky's Whey. In order to differentiate between 

 acid and alkali 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.c. are 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 found 

 clouded and acid in reaction. It is therefore filtered again, 

 and again neutralized. Litmus is finally added to the neu- 

 tral liquid, so that it has a violet color, 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 alkali formation 

 of the former and acid of the latter. 



Peptone Solution, or Dunham's solution, is useful for the 

 detection of certain faint colors. It is a perfectly clear, 

 colorless solution, made as follows: 



Sodium chlorid 0.5 



Witte's dried peptone 1.0 



Water 100.0 



Boil until the ingredients dissolve ; filter, fill into tubes and sterilize. 



It was for a long time used for the detection of indol. 

 Garini * found that many of the peptones upon the market 

 were impure, and on this account failed to show the indol 

 reaction in cultures of bacteria known to produce it. He 

 recommends testing the peptone to be employed by the use 

 of the biuret reaction. The reagent employed is Fehling's 

 copper solution, with which pure peptone strikes a violet 

 color not destroyed upon boiling, while impure peptone 

 gives a red or reddish- yellow precipitate. Both the peptone 

 and copper solutions should be in a dilute form to make 



* "Centralbl. f. Bakt. u. Parasitenk.," xm, p. 790. 



