JSiifBf* 



200 Cultivation of Micro-organisms 



the potato and keep it up out of the water. It is not necessary to 

 have a special small chamber blown in the tube to contain this water, 

 only a small quantity of which need be added. The special reservoir 

 increases the trouble of cleaning the tubes. 



If the work to be done with potatoes is to be accurate, it is nec- 

 essary to correct their variable reaction, especially if the acids have 

 not been sufficiently removed by the washing in running water 

 already described. 



To do this the cut cylinders are placed in a measured quantity 

 of distilled water and steamed for about an hour. The reaction of 

 the water is then determined by titration and the desired amount 

 of sodium hydroxid added to correct the reaction, after which the 

 potatoes are steamed in the corrected solution for about thirty 

 minutes before being placed in the tubes. " 



A potato-juice has also been suggested, and is of some value. 

 It is made thus: To 300 cc. of water 100 grams of grated potato are 

 added, and allowed to stand on ice over night. 

 Of the pulp, 300 cc. are expressed through a 

 cloth and cooked for an hour on a water bath. 

 After cooking, the 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 

 Fig. 39.— Ravenel's ^ . , -4. ■ u 4. ^ c \. ■^^ 



potato cutter. ot 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 alkaUnityby 



titration, dispensed in sterile tubes, and sterilized by steam by the 



intermittent method or in the autoclave. The opaque nature of 



this culture-medium 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 pure reagent 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 sterilized 

 and treated like the culture media. 



An excellent method of preparing reagent litmus from litmus cubes 

 is given by Prescott and Winslow* 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 



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

 126. 



