50 ESSENTIALS OF BACTERIOLOGY 



other media, must be carefully neutralized with a saturated 

 solution of sodium carbonate. Since too much alkalinity is 

 nearly as bad as none at all, the soda must be added drop by 

 drop until red litmus-paper commences to turn blue. The 

 bouillon is then cooked another hour, and filtered when cold. 

 The liquid thus obtained must be clearly alkaline, and not 

 clouded by further cooking. If cloudiness occur, the white 

 of an egg and further boiling will clear the same. To make 

 bouillon, beef-extract can be used instead of fresh meat, 2 grams 

 to i liter of water. This is boiled with 5 grams of salt and 10 

 of peptone, neutralized as above, and filtered when cold. 



Schultz's Method of Neutralization. A more accurate 

 method of obtaining the required reaction is to use an alcoholic 

 solution (J per cent.) of phenolphthalein as an indicator; a 

 few drops of this are mixed with 10 c.c. of the bouillon and 

 40 c.c. of water, and heated to boiling and while hot, ^ normal 

 sodium hydroxid from a buret is added, drop by drop, until a 

 faint pink color appears. An average is taken from three 

 different samples, and the amount of soda needed for the 

 entire quantity of bouillon is calculated therefrom, and is 

 added in the form of normal soda solution. The reaction 

 is then expressed by the + sign if acid and the sign if 

 alkaline. 



A bouillon that reacts neutral to litmus is, on an average, 

 + 25 to phenolphthalein, i. e., requires 25 c.c. of normal 

 soda solution to a liter to make it neutral to the latter indi- 

 cator. 



An optimum reaction is one about midway between the 

 neutral point of litmus and the neutral point of phenol- 

 phthalein. 



Glucose broth, which is a good medium for anaerobic organ- 

 isms, consists of bouillon to which i to 2 per cent, of grape- 

 sugar has been added. Glycerin broth is bouillon to which 6 to 

 8 per cent, of glycerin has been added after filtration. 



Sterilization of the Bouillon. Erlenmeyer flasks (little 

 conic glass bottles) or test-tubes plugged and properly sterilized 

 are filled one-third full with the bouillon, and placed with 



