62 MANUAL OF BACTERIOLOGY. 



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 



N 

 filtered. Enough — 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 



N 

 of acidity or alkalinity may be indicated by the amount of solution required 



to render the medium neutral to phenolphthalein, thus + i-S signifies that a 



N 

 medium is acid, and requires 1.5 per cent, of sodmm hydroxide to neu- 

 tralize it. 



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

 agreement 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,* 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-jree Bouillon. — Ordinary bouillon often contains some muscle- 

 sugar, which is objectionable if fermentation tests with lactose or saccharose 

 are to be made. Muscle sugar must also be removed from the beef-juice in 

 preparing diphtheria for the production of antitoxine. To secure bouillon 

 free of sugar, be^f-infusion is prepared from meat, and is inoculated in the 

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

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

 is prepared as usual.f 



Nutrient Gelatin. 



Beef-extract 3 grams. 



Peptone 10 grams. 



Sodium chloride 5 grams. 



Gelatin (best gold label) 100 grams. 



Water i liter. 



Dissolve the ingredients in the water, stirring actively to 

 prevent burning at the bottom. It is best to conduct the opera- 



* Dextrose is the principal ingredient of commercial grape-sugar or glucose 

 and should be obtained in a pure condition. 



t See Theobald Smith. Journal oj Experimental Medicine. Vol. II., p. 546. 



