74 MANUAL OF BACTERIOLOGY. 



of y sodium hydroxide needed to neutralize the whole quantity of 

 bouillon. This should be added. The bouillon should then be boiled 

 for ten minutes, and again titrated. It will usually be found acid. 

 The deficiency should be corrected by adding the necessary amount of 

 y sodium hydroxide. It should be boiled again, and again titrated, 

 and any deficiency made good. It is rarely necessary to repeat the 

 process, except to determine that the neutral point has been reached. 

 After neutralizing it is boiled thirty minutes and filtered. Enough y 

 hydrochloric acid or sodium hydroxide is added to give the degree of 

 acidity or alkalinity desired. It is then sterilized. 



An acid reaction may be denoted by +, an alkaline by . The 

 degree of acidity or alkalinity may be indicated by the amount of 

 solution required to render the medium neutral to phenolphthalein, thus 

 + 1.5 signifies that a medium is acid, and requires 1.5 per cent, of y 

 sodium hydroxide to neutralize it. 



A reaction of +1.5 is recommended as the optimum. There is much 

 disagreement as to what reaction is most favorable for the growth of 

 the majority of species of bacteria. In any case the degree of reaction 

 should be noted in descriptions. 



Bouillon may be modified by the addition to it of other 

 substances, the most important of which are glycerine (6 per 

 cent.) and sugars, as dextrose, 1 saccharose or lactose (i 

 per cent.). It is better to sterilize media containing sugars 

 in the steam sterilizer by the fractional method than in the 

 autoclave, where decomposition of the sugars may occur. 



Dextrose-free Bouillon. Ordinary bouillon often contains some 

 muscle-sugar, which is objectionable if fermentation tests with lactose 

 or saccharose are to be made. To secure bouillon free of sugar, beef- 

 infusion is prepared from fresh meat, and is inoculated in the evening 

 with a quantity of bacillus coli commnnis, and kept in the incubator. 

 Early next morning it is boiled, filtered, peptone and salt added, and 

 the bouillon is prepared as usual. 2 



Nutrient Gelatin. 



Beef-extract 3 grams. 



Peptone 10 grams. 



Sodium chloride 5 grams. 



Gelatin (best gold label) 100 grams. 



\Yuter i liter. 



1 Dextrose is the principal ingredient of commercial grape-sugar and 

 should be obtained in a pure condition. 



2 See T. Smith, Journal of Experimental Medicine, Vol. II., p. 546. 



