REPORT OF COMMITTEE ON DESCRIPTIVE CHART 129 



for beef-extract agar. Modification of these routine formula is 

 often necessary, as explained below (see p. 130). 



Starch agar. Add 0.2 per cent of water-soluble starch to the 

 ordinary beef-extract agar. 



Indicator media. Saccharine media with some indicator to 

 show acid production are frequently used. Litmus is the most 

 common indicator, enough of which should be added in saturated 

 aqueous solution to give the medium a distinct blue color (taking 

 care that the litmus solution used is not so alkaline as to alter, 

 appreciably, the reaction of the medium). Litmus, however, 

 does not give accurate results in terms of hydrogen ion concen- 

 tration; so except for certain special purposes (see p. 130) it is 

 recommended that brom cresol purple be used to detect increase 

 in acidity, and cresol red to detect increase in alkalinity. It is 

 convenient to keep these indicators in concentrated alcoholic solu- 

 tions of such strength that 1 cc. will be sufficient for each litre of 

 medium. For this purpose a 1.6 per cent solution of either indi- 

 cator in 95 per cent alcohol is recommended. Brom cresol pur- 

 ple is purple in neutral or alkaline media and is yellow in acid 

 media. Cresol red is yellow at neutrality and in acid media, 

 turning red under alkaline conditions. Under certain circum- 

 stances it is desirable to have an indicator that will show a change 

 in either direction from neutrality. Brom thymol blue does this, 

 but is not satisfactory in media because its range is too short to 

 distinguish differences between different kinds of bacteria. Very 

 satisfactory results, however, may be obtained with a mixture of 

 brom cresol purple and cresol red (0.5 cc. each of 1.6 per cent 

 alcoholic solution of the two dyes to the litre of medium.) This 

 mixture changes very slowly from purple to yellow through a 

 long range (from about pH = 8.0 to about pH = 5.0) extending 

 to a considerable distance on both sides of neutrality. By com- 

 paring with a blank tube of the neutral medium, it is very easy 

 to detect an increase in either acidity or alkalinity. 



Recently certain other combinations of indicators have been 

 recommended for this same purpose. Bronfenbrenner (1919) 

 recommends a combination of china blue with rosolic acid or 

 preferably its sodium salt, and Morishima (1920) a combination 



