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

Abstract

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 CH = 1 × 10-9 and CH = 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 CH = 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 COH = 1 × 10-5 ...