The production of acids lowers the pH of the medium. After incubation, a methyl red indicator is added to the medium for the mixed acid production test. If the color changes to red, then the test is said to be positive for mixed acid production, if the color changes to orange, the result is inconclusive and is incubated further and if the color does not change at all , then the result is said to be negative without production of mixed acids (Leboffe and Pierce