Litmus Milk 239 



liquid is filtered, titrated if desired, and receives an addition 

 of 4 per cent, of glycerin. Upon this medium the tubercle 

 bacillus grows well, especially when the reaction of the 

 medium is acid. 



Milk. Milk is a useful culture medium. As the cream 

 which rises to the top is a source of inconvenience, it is best 

 to secure fresh milk from which the cream has been removed 

 by a centrifugal machine. It is given the desired degree of 

 alkalinity by titration, dispensed in sterile tubes, and ster- 

 ilized by steam by the intermittent method or in the auto- 

 clave. The opaque nature of this culture medium often 

 permits the undetected development of contaminating or- 

 ganisms. 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 receptacle 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 

 exhausted 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 ^ 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 ^r HC1 

 should turn it red, one drop of ^ NaOHO should turn it 

 blue. Filter the solution and sterilize at noC. This 

 solution should be added to the media just before use in 

 the proportion of about \ c.c. to 5 c.c. of medium. 



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

 1904, p. 126. 



