ON METHODS OF ISOLATION AND IDENTIFICATION 

 OF THE MEMBERS OF THE COLON-TYPHOID 

 GROUP OF BACTERIA 



STUDY OF THE BACTERICIDAL ACTION OF CR INDICATOR* 



J. BRONFENBRENNER, M. J. SCHLESINGER and D. SOLETSKY 



From the Department of Preventive Medicine and Hygiene of the Harvard Medical 

 School, Boston, Massachusetts 



Received for publication August 6, 1919 



In the course of a study of the fermentation of carbohydrates 

 by the members of the colon-typhoid group, we have tried to find 

 an indicator which could be added to the medium to permit 

 direct reading of the hydrogen ion concentration during the prog- 

 ress of fermentation, without necessitating the destruction of 

 the culture. A mixture of China blue and Rosolic acid (CR) 

 seemed to answer this purpose. 



Since the time of our first publication on this subject, we have 

 successfully continued to use CR as indicator in our bacterio- 

 logical studies, and have made several observations some of 

 which are of sufficient importance to deserve special mention. 

 In our original description of this indicator (Bronfenbrenner, 

 1918) it was stated that its great tinctorial powers permit the 

 use of very dilute solutions of the dyes in the medium, and for 

 this reason the bactericidal properties of the dyes can be reduced 

 to a negligible quantity. So long as we continued working with 

 the bacteria of the colon-typhoid group we found this to be the 



1 This work is a part of the investigation of food poisoning, conducted under 

 the Direction of Dr. M. J. Rosenau, Professor of Preventive Medicine and 

 Hygiene, Harvard Medical School. The investigations were made under the 

 auspices of the Advisory Committee of the National Research Council on the 

 Toxicity of Preserved Foods, and under a grant to Harvard University from 

 the National Canners Association. 



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