XLDA (Xylose Lysine Deoxycholate Agar): Composition, Principle, and Applications in Microbiology

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XLDA (Xylose Lysine Deoxycholate Agar): Composition, Principle, and Applications in Microbiology

Xylose Lysine Deoxycholate Agar (XLDA) is a selective and differential culture medium primarily used for the isolation and differentiation of Salmonella and Shigella species from clinical, food, and pharmaceutical samples. It is designed to inhibit the growth of most Gram-positive organisms while allowing selective recovery of enteric pathogens from mixed bacterial populations.

💡 Principle of XLDA

XLDA is based on the ability of enteric organisms to ferment carbohydrates and decarboxylate lysine. It contains xylose, lactose, and sucrose as fermentable carbohydrates. Most enteric bacteria, except Shigella, can ferment xylose — turning the medium yellow due to acid production. After xylose depletion, Salmonella decarboxylates lysine, re-alkalizing the medium to a red color. The presence of sodium deoxycholate inhibits Gram-positive bacteria, while ferric ammonium citrate and sodium thiosulfate allow detection of hydrogen sulfide (H2S) production.

🧪 Composition of XLDA (per liter)

IngredientQuantity (g/L)
Xylose3.75
L-Lysine5.00
Lactose7.50
Sucrose7.50
Sodium Chloride5.00
Yeast Extract3.00
Sodium Thiosulfate6.80
Ferric Ammonium Citrate0.80
Sodium Deoxycholate2.50
Phenol Red0.08
Agar13.50
Final pH (at 25°C)7.4 ± 0.2

⚗️ Preparation of XLDA Medium

  1. Weigh 65.93 g of XLDA powder and suspend it in 1 liter of distilled water.
  2. Heat with gentle agitation until completely dissolved.
  3. Do not autoclave; instead, boil gently for one minute to sterilize.
  4. Cool to around 50–55°C and pour into sterile Petri plates.
  5. Once solidified, store the plates in a refrigerator at 2–8°C until use.

🔬 Principle of Selectivity and Differential Action

  • Sodium deoxycholate – inhibits Gram-positive bacteria, ensuring selectivity for Gram-negative enteric organisms.
  • Phenol red – serves as a pH indicator; yellow for acidic (fermentation), red for alkaline reactions.
  • Ferric ammonium citrate and sodium thiosulfate – detect hydrogen sulfide (H2S) production, forming black-centered colonies in Salmonella.
  • L-Lysine – differentiates Salmonella (lysine positive) from Shigella (lysine negative).

🧫 Colony Characteristics on XLDA

OrganismColony Appearance
Salmonella spp.Red colonies with or without black centers (H2S positive)
Shigella spp.Red colonies without black centers
Escherichia coliYellow colonies due to lactose/sucrose fermentation
Proteus spp.Red to black colonies; may produce H2S

📘 Applications of XLDA Medium

  • Isolation and differentiation of Salmonella and Shigella species from stool, food, and water samples.
  • Used in pharmaceutical microbiology for microbial limit testing.
  • Quality control analysis in food and beverage industries.
  • Routine use in public health and environmental laboratories for enteric pathogen detection.

🧾 Quality Control

Test OrganismExpected Growth
Salmonella Typhimurium ATCC 14028Red colonies with black centers
Shigella flexneri ATCC 12022Red colonies without black centers
Escherichia coli ATCC 25922Yellow colonies
Enterococcus faecalis ATCC 29212Completely inhibited

⚠️ Precautions

  • Do not overheat or autoclave the medium, as it may destroy selectivity.
  • Prepare fresh medium before use to maintain color and pH stability.
  • Always confirm suspicious colonies using biochemical or serological tests.

✅ Conclusion

XLDA (Xylose Lysine Deoxycholate Agar) plays a crucial role in the selective isolation of Salmonella and Shigella species. Its ability to distinguish enteric pathogens based on carbohydrate fermentation, lysine decarboxylation, and hydrogen sulfide production makes it one of the most valuable media in clinical, food, and pharmaceutical microbiology.

💬 About the Author

Siva Sankar is a Pharmaceutical Microbiology Consultant and Auditor with extensive experience in sterility testing, validation, and GMP compliance. He provides consultancy, training, and documentation services for pharmaceutical microbiology and cleanroom practices.

📧 Contact: siva17092@gmail.com
Mobile: 09505626106

📱 Disclaimer: This article is for educational purposes and does not replace your laboratory’s SOPs or regulatory guidance. Always follow validated methods and manufacturer instructions.

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