84 J. BRONFENBRENNER, M. J. SCHLESINGER AND D. SOLETSKY 



case. As soon, however, as we attempted to use this indicator 

 in connection with other bacteria, we found that it exhibited de- 

 cided antiseptic action for some of them, even when used in lower 

 concentration than that which was found to be non-inhibitory for 

 the colon group. 



This difference in the relative sensitiveness of various bacteria 

 to the antiseptic action of CR at once suggested the necessity of 

 further study of this indicator if it is to be incorporated in the 

 culture medium. 



METHOD OF STUDY 



With this end in view, a number of bacteria were tested against 

 various concentrations of China blue, Rosolic acid and CR (which 

 is a mixture of the two) , in order to determine the respective con- 

 centrations affecting growth. In all 78 strains were tested. Of 

 these, 43 were pathogenic members of the Colon-Typhoid-Dys- 

 entery group; B. typhosus, B. paratyphosus A and B, B. dysenter- 

 eae Shiga and Flexner, B. enteritidis and B. suipestifer, a number 

 of strains of each being tested. The rest of the cultures, 35 in 

 number, were cocci of various types, Gram positive spore bearers, 

 B. coli, B. proteus, B. cloacae, B. fluoresceins, B. alkaligenes, and 

 several strains of an unidentified Gram negative bacillus. Most 

 of these non-pathogenic strains were freshly isolated by us espe- 

 cially for this study, from feces and other sources, and have not 

 been definitely identified. A few were obtained from the Museum 

 of Natural History. 



The China blue used was manufactured by Grubler and the 

 Rosolic acid by Merck. Stock solutions were made up in 50 per 

 cent alcohol, and contained respectively 1 per cent China blue, 

 2 per cent Rosolic acid and in the CR mixture 1 per cent China 

 blue plus 2 per cent Rosolic acid. 



The medium used was the same in all cases, a solution of 1 per 

 cent Difco peptone and 0.5 per cent chemically pure sodium chlor- 

 ide in distilled water. The dyes in alcoholic solution were added 

 in appropriate amounts to this medium which was then placed in 



