Thermo Scientific™ Oxoid™ Simmons Citrate Agar (Dehydrated)

Thermo Scientific™ Oxoid™ Simmons Citrate Agar is a selective diagnostic medium used to differentiate members of the family Enterobacteriaceae based on their ability to utilize citrate as the sole carbon source. This agar, a solidified modification of Koser's citrate medium, is enhanced with bromothymol blue as a pH indicator, making it easier to identify citrate utilization with a distinct color change.

Key Features

1. Selective Differentiation: Differentiates between citrate-positive and citrate-negative members of Enterobacteriaceae.
2. Distinct Color Indication:
   • Positive Test: Bright blue medium indicates citrate utilization with an alkaline reaction.
   • Negative Test: Medium remains green, indicating no citrate utilization.
3. Enhanced Accuracy: Solidified formulation with agar prevents false growth appearances caused by excessive inoculum, a limitation of liquid Koser citrate medium.
4. Wide Application: Suitable for use in slopes (test tubes) or plate cultures (petri dishes).
5. Compliance: Meets recommendations of the American Public Health Association (APHA).

Applications

• Clinical Microbiology: Differentiates enteric pathogens such as Salmonella enterica subspecies based on citrate utilization:
  • Citrate-positive: Salmonella enteritidis, Salmonella subgenus II-IV.
  • Citrate-negative: Salmonella typhi, Salmonella paratyphi A, Salmonella pullorum, Salmonella gallinarum.
• Bacterial Identification: Useful in distinguishing genera such as:
  • Citrate-positive: Enterobacter, Klebsiella, Proteus, Citrobacter, Serratia, Providencia.
  • Citrate-negative: Escherichia coli, Shigella spp., Edwardsiella spp., Yersinia spp.
• Quality Control: Detects citrate utilization for validation of microbiological assays.

Composition

• Citrate: Sole carbon source.
• Ammonium phosphate: Sole nitrogen source.
• Bromothymol Blue: pH indicator that changes from green (neutral) to blue (alkaline).
• Agar: Solidifying agent to prevent false-positive results.

Test Procedure

1. Preparation: Dissolve the dehydrated medium in distilled water and autoclave as instructed on the product label.
2. Inoculation: Streak the test organism onto a slant or plate of Simmons Citrate Agar.
3. Incubation: Incubate at 35–37°C for 24–48 hours.
4. Interpretation:
   • Positive: Growth with a blue color change.
   • Negative: No growth; medium remains green.

Results Interpretation

• Positive Growth:
  • Indicates the ability to utilize citrate, leading to an alkaline pH (blue medium).
  • Examples: Klebsiella pneumoniae, Enterobacter cloacae, Salmonella enteritidis.
• Negative Growth:
  • Indicates the inability to utilize citrate; medium remains green.
  • Examples: Escherichia coli, Shigella flexneri, Salmonella typhi.

Advantages

1. Clear Differentiation: Easily distinguishes citrate-positive from citrate-negative organisms.
2. High Specificity: Supports accurate bacterial identification in the Enterobacteriaceae family.
3. Multiple Formats: Can be used in both slants and plates for diverse laboratory needs.

Limitations

• Some strains of Morganella morganii and Klebsiella rhinoscleromatis do not utilize citrate and may yield false-negative results.
• Incubation conditions and inoculum size must be carefully controlled to avoid inconsistent results.

Storage and Stability

• Store the dehydrated medium in a cool, dry place at 2–8°C.
• Prepared plates/slants can be stored for short periods under refrigeration (consult product label for shelf life).

Thermo Scientific™ Oxoid™ Simmons Citrate Agar is an essential tool for differentiating enteric bacteria based on citrate utilization, providing reliable results for clinical, food, and environmental microbiology applications.