Peptone Solution 201 



of water until 3 to 4 liters of water have been used and the cubes are well ex- 

 hausted of coloring matter. Pour the decantations together and allow them to 

 settle over night. Siphon off the clear solution. Concentrate to about i liter and 

 make the Solution decidedly acid with glacial acetic acid. Boil down to about 

 }i liter and make exactly neutral with caustic soda or potash. To test for the 



neutral point, place one drop of the solution in a test-tube, while one drop of — 



HCl should tiurn it red, one drop of — NaOHO should turn it blue. Filter the 



20 



solution and sterilize at ito°C. This solution should be added to the media just 

 before use in the proportion of about H cc. to s cc. of medium. 



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 sepa- 

 rately and pour them together subsequently. It is said that lac- 

 moid is never thus changed, and many workers prefer it to litmus on 

 that account. 



Petrusch^'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 Hter of fresh skimmed milk i liter of water is added. The 

 mixture is violently shaken. About 16 cc. 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 boihng it is 

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

 and again neutralized. Litmus is finally added to the neutral liquid, 

 so that it has a violet color, changed to blue or red by alkalies or 

 acids. 



Peptone Solution, or Dunham's solution, is a perfectly clear, 

 colorless solution, made as follows: 



Sodium chlorid 0.5 



Witte's dried peptone i.o 



Water loo.o 



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 bac- 

 teria known to produce it. He recommends testing the peptone to 

 be employed by the use of the biuret reaction. The reagent em- 

 ployed 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 successful tests. 



The addition of 4 cc. of the following solution — 



Rosolic acid 0.5 



Eighty per cent, alcohol loo.o 



* II 



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



