Peptone Solution 199 



dium often permits the undetected development of contaminating 

 organisms. A careful watch should therefore be kept lest it spoil. 



Litmus Milk. — This is milk to which just enough of a saturated 

 watery solution of pulverized litmus is added to give a distinct blue 

 color after titration. Litmus milk is probably the best reagent for 

 determining acid and alkali production by bacteria. 



The watery solution of litmus, being a vegetable infusion, is likely 

 to be spoiled by micro-organismal growth, hence must be treated 

 like the culture media and sterilized by steam every time the recep- 

 tacle in which it is kept is opened. 



An excellent method of preparing litmus is given by Prescott and 

 Winslow* and is as follows: 



To one-half pound of litmus cubes add enough water to more than cover, boil, 

 decant off the solution. Repeat this operation with successive small quantities 

 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 iliter and 

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

 J^ 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 — 



n ^° 



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



20 



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



before use in the proportion of about 34 cc. to 5 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. 



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 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 boiling 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 100 . o 



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



* "Elements of Water Bacteriology," John Wiley & Sons, New York, 1904, p. 

 126. 



