The negative result is indicated by no color change in the oil-covered tube and in some cases an increase in pH (pH 7.6) changing the bromthymol blue from green to blue (6, 8) in the top of the open tube. The increase in pH is due to amine production by bacteria that break down the peptone (protein) in the medium (1, 5). Other bacteria give a negative result indicated by no growth or color change in the medium (5,