What is a positive reaction for the Simmons Citrate agar slant?

When Simmons Citrate agar is inoculated with Salmonella typhimurium , the medium turns royal blue. This is a positive result for the citrate test. When Simmons Citrate agar is inoculated with Escherichia coli , the medium remains green. This is a negative result for the citrate test.

What does a positive result indicate in the citrate test?

A positive result is color change from green to blue and/or growth. This means that citrate is utilized. Bacteria that use citrate converts the ammonium which alkalizes the agar. Therefore the dye changes from pH of 6.9 to 7.6 (acidic to alkaline) and the green dye indicates the pH going up by its color change to blue.

What color does the citrate test turn if it is positive?

Result Interpretation of Citrate Utilization Test Positive Reaction: Growth with color change from green to intense blue along the slant. Examples: Salmonella, Edwardsiella, Citrobacter, Klebsiella, Enterobacter, Serratia, Providencia, etc. Negative Reaction: No growth and No color change; Slant remains green.

What does the Simmons citrate test demonstrate?

Citrate is the only carbon source in both Koser’s and Simmons media, thus growth in either medium indicates that the test organism has the ability to use citrate as its sole carbon source.

Why does a positive Simmons Citrate agar result in the slant turning blue?

When the bacteria metabolize citrate, the ammonium salts are broken down to ammonia, which increases alkalinity. The shift in pH turns the bromthymol blue indicator in the medium from green to blue above pH 7.6.

Which organism is positive for citrate test?

Klebsiella pneumoniae and Proteus mirabilis are examples of citrate positive organisms. Escherichia coli and Shigella dysenteriae are citrate negative.

What bacteria test positive for citrate?

What is the purpose of Simmons Citrate agar?

Simmons’ citrate agar is used for differentiating gram-negative bacteria on the basis of citrate utilization. It is useful for selecting for organisms that use citrate as its main carbon and energy source.

What makes citrate agar selective?

The citrate utilization test is selective because only certain bacteria can utilize citrate in place of a fermentable carbohydrate.

What is the purpose of the citrate test?

Citrate testing is used to determine the ability of the bacteria to use sodium citrate as the only source of carbon and inorganic ammonium hydrogen phosphate (NH4H2PO4) as a source of nitrogen. The citrate utilization test is possible only if the organisms are capable of fermenting citrate.

Why is Simmons Citrate a selective medium?

Simmons citrate agar is a selective and differential medium that tests for an organism’s ability to use citrate as a sole carbon source and ammonium ions as the sole nitrogen source. It is used for differentiating gram-negative bacteria on the basis of citrate utilization.