What does Simmons citrate test for?
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.
Is E coli citrate positive?
Klebsiella pneumoniae and Proteus mirabilis are examples of citrate positive organisms. Escherichia coli and Shigella dysenteriae are citrate negative.
What are the expected color reactions of Bromothymol blue in Simmon citrate medium with different pH?
At pH 7.5 or above, bromthymol blue turns royal blue. At a neutral pH, bromthymol blue is green, as evidenced by the uninoculated media. If the medium turns blue, the organism is citrate positive. If there is no color change, the organism is citrate negative.
Why does the pH of Simmon’s citrate agar increase if citrate is utilized?
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.
What does a positive result for a citrate test look like?
A positive test would be indicated by a color change in the medium from yellow at pH 6.7 to red at more alkaline pH values. A citrate-negative culture would remain yellow. Christensen’s medium can also be used to test for the generation of hydrogen sulfide (H2S) by the organism.
What is the positive result for citrate 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.
How do you read citrate test results?
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 a positive results 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.
Which medium is used in citrate utilization test?
Citrate agar is used to test an organism’s ability to utilize citrate as a source of energy. The medium contains citrate as the sole carbon source and inorganic ammonium salts (NH4H2PO4) as the sole source of nitrogen.
Which enterobacteria is used as a +ve control in citrate utilization test?
Citrate utilization can be used to distinguish between coliforms such as Klebsiella (formerly Enterobacter) aerogenes (+ve) which occur naturally in the soil and in aquatic environments from fecal coliforms such as Escherichia coli (-ve) whose presence would be indicative of fecal contamination.