A positive citrate test indicates that a microorganism can utilize citrate as its sole source of carbon for growth and metabolism.
Here's a breakdown of what that means and how it's observed:
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Citrate Utilization: The test specifically assesses the ability of an organism to transport citrate into the cell using the enzyme citrate permease. Once inside, citrate is broken down.
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Simmons Citrate Agar: The test is typically performed using Simmons citrate agar. This medium contains citrate as the only available carbon source and ammonium phosphate as the only available nitrogen source. It also contains a pH indicator, bromothymol blue.
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Positive Result - Color Change: If the organism can utilize citrate, it will also utilize ammonium phosphate. This utilization breaks down the ammonium phosphate, producing ammonia (NH3) which raises the pH of the medium, making it alkaline. The bromothymol blue indicator responds to this increase in pH by changing from green (neutral) to blue (alkaline). A blue color is a positive result.
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Positive Result - Growth: Even if the medium doesn't turn blue, visible growth on the slant indicates a positive result. The organism may be slowly utilizing citrate, resulting in insufficient pH change for a dramatic color shift, but still enough to support growth. Therefore, growth alone, even without a color change from green to blue, is considered a positive test.
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Negative Result: If the organism cannot utilize citrate, it will not grow on the medium. The slant will remain green and there will be no visible growth.
In summary, a positive citrate test is indicated by:
- A blue color change on the Simmons citrate agar slant.
- Visible growth on the slant, even without a color change.
