This image from 1975, depicts a spot plate containing three rows of specimen wells. The closest row, i.e., Row 1, contains an unknown culture specimen with no added indicator. Row 2, or the middle row wells contain culture specimens, to which Bromthymol Blue (bromothymol blue, BTB) indicator had been added. At right, you can see the partially filled bottle of indicator with its dropper top applicator. BTB can be used as an indicator when a change in pH is suspected. Under acidic conditions, where the pH is less than 7.0, the indicator will display a yellow coloration, and under alkaline, or basic conditions, where a pH greater than 7, BTB will display a blue coloration. Under neutral conditions, where the pH is 7.0, BTB will display a gray-green coloration.