BOUILLON 109 



It is then filtered through a folded paper filter and sterilized 

 by steam. Certain modifications of this method are of 

 sufficient value to justify mention. Most important is the 

 neutralization. 



In the exhaustive paper of Fuller 1 on the question of 

 reaction it was shown that the results obtained by titrating 

 the same culture-medium with the same alkaline solution 

 differed very markedly with the indicator employed. For 

 instance, 1 liter of ordinary meat-infusion nutrient agar- 

 agar required 47 c.c. of a normal caustic alkali solution to 

 neutralize it when phenolphthalein was the indicator used, 

 28 c.c. when blue litmus was employed, and 5 c.c. when rosolic 

 acid was substituted. It is manifest from this that the 

 actual reactions of media, in the neutralization of which 

 different indicators have been used, may differ very widely 

 from one another, and that the results of cultivation on a 

 medium neutralized by one method are not fairly comparable 

 with those obtained when another indicator has been used. 

 For the sake of uniformity Fuller suggests that bacteriolo- 

 gists should agree upon some one trustworthy method of 

 neutralization and employ it to the exclusion of other 

 methods. He recommends, as the procedure that has given 

 the most satisfactory results in his hands, a modification 

 of Schultz's method, viz., 5 c.c. of the culture-medium are 

 to be mixed with 45 c.c. of distilled water in a porcelain 

 evaporating-dish and boiled for three minutes, after which 

 1 c.c. of phenolphtalein solution 2 is added and titration with 

 the one-twentieth normal caustic alkali solution is quickly 

 made. The neutral point (slightly on the side of alkalinity) 



1 On the Proper Reaction of Nutrient Media for Bacterial Cultivation, 

 Public Health (Journal of the American Public Health Association), Quar- 

 terly Series, 1895, vol. i, p. 381. 



2 A 0.5 per cent, solution of the powder in 50 per cent, alcohol. 



