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Simmons Citrate Agar Defined Medium for Citrate Utilization Testing
  • Simmons Citrate Agar Defined Medium for Citrate Utilization Testing
Simmons Citrate Agar (AS-1348)

Simmons Citrate Agar (AS-1348)

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AuSaMicS Life Science • Biochemical Differentiation Media

Simmons Citrate Agar

Defined Medium for Citrate Utilization Testing

Simmons Citrate Agar is a defined differential medium used to determine whether microorganisms can utilize citrate as the sole carbon source and ammonium salts as the sole nitrogen source. It is widely used in IMViC testing and routine biochemical differentiation of Enterobacteriaceae and other Gram-negative bacteria.

AS-1348 Defined Citrate Medium Australian Stock IMViC Panel Ready
Panel: IMViC & extended biochemical ID Readout: Green to blue indicator shift Use: Citrate utilization profiling

Citrate Utilization Specialist

Defined formula for reproducible results
Bromothymol blue for easy interpretation
Ideal for IMViC workflows
Teaching and QC friendly
Routine diagnostic utility
Indicator System
Alkaline citrate metabolism shifts the medium toward blue

Overview & Applications

Simmons Citrate Agar is used to determine the ability of microorganisms to utilize citrate as the sole carbon source and ammonium phosphate as the sole nitrogen source. Organisms capable of citrate metabolism produce alkaline by-products that shift the pH indicator from green toward blue.

The medium is classically incorporated into IMViC testing and broader biochemical identification schemes for enteric and related Gram-negative organisms.

Primary Applications
  • Differentiation of Enterobacteriaceae
  • IMViC “C” component
  • Clinical, food, and environmental microbiology
  • Teaching practicals and QC workflows
Key Features
  • Defined mineral salts medium
  • Clear bromothymol blue endpoint
  • Useful in slant format for easy reading
  • Supports standardized biochemical identification

Defined Simmons Citrate Formulation

Base Medium Composition (per Liter)

Component Amount Function Diagnostic Role
Sodium Citrate 2.0 g Sole carbon source Selects for organisms able to transport and metabolize citrate
Ammonium Dihydrogen Phosphate 1.0 g Sole nitrogen source Supports growth only when citrate metabolism is active
Dipotassium Phosphate 0.2 g Buffer Helps maintain indicator performance
Magnesium Sulfate 0.2 g Cofactor source Supports enzymatic activity
Sodium Chloride 5.0 g Osmotic balance Maintains ionic environment
Bromothymol Blue 0.08 g pH indicator Changes from green to blue as alkaline metabolites accumulate
Agar 15.0 g Solidifying agent Provides slant surface for visual interpretation
Final pH: 6.8 ± 0.2 at 25°C

Preparation, Incubation & Interpretation

Preparation

  1. Suspend 18.4 g of dehydrated medium in 1 L purified or distilled water.
  2. Heat with gentle agitation until fully dissolved.
  3. Dispense into tubes for slants or suitable vessels for plates.
  4. Autoclave at 121°C for 15 minutes.
  5. Cool in a slanted position when preparing citrate slants.

Incubation Conditions

  • Temperature: 35–37°C
  • Duration: 24–48 hours
  • Weak or slow reactions may require extended incubation
  • For some organisms, observation may continue up to 7 days

Result Interpretation

  • Positive: visible growth with blue coloration or strong blue-green shift
  • Negative: no growth and medium remains green
  • Growth without obvious color change should be interpreted carefully and confirmed
  • Always assess alongside the broader biochemical profile

Biochemical Panel Integration

Medium / Test Primary Target Principle Typical Use Key Advantage
Simmons Citrate Agar (AS-1348) Gram-negative rods / Enterobacteriaceae Citrate utilization IMViC “C” test Defined composition and clear endpoint
Indole Medium Tryptophanase-positive organisms Indole production IMViC “I” Differentiates E. coli from many others
Methyl Red Broth Mixed-acid fermenters Stable acid production IMViC “MR” Detects strong acid fermentation
Voges-Proskauer Broth Acetoin producers Neutral end-product detection IMViC “VP” Useful for Enterobacter / Klebsiella differentiation
TSI / KIA Slants Enteric organisms Sugar fermentation, gas, H₂S Supplementary enteric ID Multiple reactions in one medium

Strategic Use of Simmons Citrate Agar

Key Advantages

  • Defined formulation for reproducibility
  • Easy visual endpoint via bromothymol blue
  • Standard IMViC medium
  • Useful in clinical, food, water, and teaching labs

Method Considerations

  • Heavy inocula can distort weak reactions
  • Some strains require longer incubation
  • Growth without clear color change may need confirmation
  • Should be read as part of the full biochemical profile

Selection Guide

  • Use in every classical IMViC workflow
  • Excellent for enteric differentiation training labs
  • Combine with TSI/KIA for broader biochemical profiling

Quality Control & Performance Standards

Positive Control:
Klebsiella pneumoniae or Enterobacter aerogenes — growth with blue coloration
Negative Control:
Escherichia coli — no growth and medium remains green
Physical QC:
Green medium; firm gel; correct pH and appearance
Documentation:
COA and lot-specific QC data available on request

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