A positive Simmons citrate test is indicated by what color change in the medium?

In the Simmons citrate test, a positive result is indicated by a change in the color of the medium to blue. This occurs because the medium contains bromthymol blue as a pH indicator, which changes color in response to alterations in the acidity or alkalinity of the environment.

When an organism is capable of utilizing citrate as its sole carbon source, it metabolizes the citrate and produces alkaline byproducts. This increase in pH causes the pH indicator to shift from its original color (green) to blue, indicating a positive result. This color change is a critical aspect of the test, as it confirms the organism's ability to utilize citrate and differentiate between different bacterial species within the Enterobacteriaceae family.

In contrast, other colors, such as red, yellow, or green, reflect the medium's original state or responses of bacteria that cannot utilize citrate, therefore solidifying that the blue color is synonymous with a positive result in this test.