26 



Scientific Proceedings (ioi). 



The combination of the two dyes thus offers an indicator which 

 has a faint gray tinge at the neutral point with pure blue and 

 pink on the acid and alkaline sides respectively. Due to the 

 high tinctorial power of the dyes composing it, this indicator is 

 incorporated into the media in very minute quantities. The 

 actual concentrations of China blue and rosolic acid in the medium 

 are respectively 0.0025 per cent, and 0.005 per cent, by weight. 



This indicator, however, can be used only for the study of the 

 Gram negative organisms, because the rosolic acid it contains 

 exerts selective bactericidal action against the Gram positive 

 organisms. 1 While this property of the rosolic acid is useful in 

 certain circumstances (as for instance, for the purpose of sup- 

 pressing Gram positive bacteria while isolating the Gram negative 

 from the mixtures of both) 2 , it limits the usefulness of the mixture 

 as an indicator. In order to permit the use of our indicator 

 in the study of Gram positive as well as Gram negative bacteria, 

 we suggest the substitution of corallin (Harmer) for the rosolic 

 acid (Merk) in the above mixture. The turning point of corallin 

 is the same as that of rosolic acid, and its tinctorial power, as 

 well as the color, are much the same: Corallin (Harmer), however* 

 has no bactericidal action upon Gram positive organisms. 



As the supply of either China blue, rosolic acid or corallin of 

 foreign manufacture is very low at present, and as the domestic 

 dyes sold under the same names are manifestly different from the 

 foreign dyes used by us, it was necessary to try a number of re- 

 lated dyes before a suitable choice could be made. While this 

 work is still in progress, we have already found a few preparations 

 which seem to answer the requirements. 



(This work is a part of the investigation of food poisoning, con- 

 ducted under the direction of Dr. M. J. Rosenau, professor of 

 preventive medicine and hygiene, Harvard Medical School. 

 The investigations are done 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.) 



1 J. Bronfenbrenner, M. J. Schlesinger and D. Soletsky. Jour. Bad. (in press). 

 ' J. Bronfenbrenner, M. J. Schlesinger and D. Soletsky, Jour. Med. Res. (in 

 press). 



