On methods of isolation and identification of the members of the colon-typhoid group of bacteria. Further studies on C. R. indicator

Some time ago we described an indicator for the direct measurement of the hydrogen ion concentration in growing bacterial cultures. 1 This indicator consists of the mixture of China blue and rosolic acid, and covers the range of changes in the hydrogen ion concentration between C_H = 1 × 10^-9 and C_H = 5 × 10^-5. The choice of the dyes was made on the basis of their possessing suitable turning points coupled with the fact that their respective phases of highest color lie on the opposite sides of the point of neutrality. China blue is colorless at the concentration of hydrogen ions below 1 × 10^-7, and gives graded intensity of blue with the increase in the hydrogen ion concentration up to the point of about C_H = 5 × 10^-5, when it reaches its maximum color. Rosolic acid, on the other hand, gives graded intensities of pink beginning with the hydrogen ion concentration close to 1 × 10^-7, and increasing gradually with the increase of the hydroxyl ions concentration reaching its maximum color at the C_OH = 1 × 10^-5 (or C_H = 1 × 10^-9). With the increase in concentration of hydrogen ions above 1 × 10^-7 the rosolic acid has a pale yellow (straw) color, which is masked by the color of ordinary culture media.

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.