Description
Simmons Citrate Agar is used for differentiation between faecal coli and members of the aerogenes group based on citrate utilization.
Simmons Citrate Agar is a selective and differential medium used in microbiology for the cultivation and differentiation of bacteria, particularly those belonging to the Enterobacteriaceae family. Named after its developer, Dr. William Simmons, this agar medium contains sodium citrate as the sole carbon source and ammonium dihydrogen phosphate as the sole nitrogen source. The citrate utilization by bacteria is a crucial test in identifying organisms like Escherichia coli and Klebsiella pneumoniae. Organisms capable of utilizing citrate as a carbon source will produce alkaline byproducts, causing the pH of the medium to rise and resulting in a color change from green to blue due to the presence of a bromothymol blue indicator. This distinct color change aids in the differentiation of citrate-positive organisms from citrate-negative ones, making Simmons Citrate Agar an invaluable tool in microbiological laboratories for bacterial identification.
Composition of Simmons Citrate Agar
- Ammonium dihydrogen phosphate: 1.000 Gms / Litre
- Magnesium sulfate: 0.200 Gms / Litre
- Dipotassium phosphate: 1.000 Gms / Litre
- Sodium citrate 2.000 Gms / Litre
- Sodium chloride 5.000 Gms / Litre
- Bromo thymol blue 0.080 Gms / Litre
- Agar 15.000 Gms / Litre
- Final pH ( at 25°C) 6.8±0.1
- Formula adjusted, standardized to suit performance parameters
Directions of Simmons Citrate Agar:
Suspend 24.28 grams in 1000 ml distilled water. Heat to boiling to dissolve the medium completely. Mix well and distribute in tubes. Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes. Cool the tubes as slants.
Principle And Interpretation
These media are used for the differentiation between Enterobacteriaceae and the members of the aerogenes group based on citrate utilization as the sole carbon source. Initially, the citrate medium was developed by Koser (1) containing ammonium salt as the only nitrogen source and citrate as the only carbon source for differentiating Escherichia coli and Enterobacter aerogenes by IMViC tests. Later on, Simmons (2) modified Koser’s formulation by adding agar and bromothymol blue (3). It is recommended by APHA (4). It is also recommended by BIS (5) for isolation of Escherichia coli Ammonium dihydrogen phosphate and sodium citrate serve as the sole nitrogen and carbon source respectively. Bromo thymol blue is the pH indicator. The organisms produce an alkaline reaction which is indicated by the change in colour from green to blue.
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