Rose Bengal Agar (RBA)- Introduction, Principle, Composition, Test Procedure, Colony Characteristics, Uses, and Keynotes

Rose Bengal Agar (RBA)- Introduction, Principle, Composition, Test Procedure, Colony Characteristics, Uses, and Keynotes

Introduction

Rose Bengal Agar (RBA) is a selective medium used for isolating fungi. It contains rose bengal dye, which inhibits bacterial growth. This medium supports the growth of many fungi. Rose Bengal Agar includes chloramphenicol, which further suppresses bacterial contamination. It is especially useful in environmental and clinical mycology. Fungal colonies appear distinct and colorful on this medium. The dye also restricts the spread of colonies, aiding in identification. Scientists use RBA to study fungal diversity. Overall, Rose Bengal Agar provides a reliable method for fungal isolation and identification in mixed cultures.

Composition and Principle

Rose Bengal Agar (RBA) from HiMedia is a selective medium designed for isolating fungi, especially from environmental samples but also useful for clinical specimens. The composition supports fungal growth while inhibiting bacterial contamination. Here is the typical composition per liter of distilled water:

  • Peptone: 5.0 g – Provides essential nutrients, including nitrogen, for fungal growth.
  • Dextrose: 10.0 g – Serves as a carbon source to support the energy needs of fungi.
  • Potato Starch: 15.0 g – Adds extra nutrients and promotes fungal growth.
  • Monopotassium Phosphate: 1.0 g – Acts as a buffer to maintain pH stability.
  • Magnesium Sulfate: 0.5 g – Provides essential minerals that support fungal metabolism.
  • Rose Bengal: 0.05 g – A dye that inhibits bacterial growth and limits the spread of fungal colonies.
  • Chloramphenicol: 0.1 g – An antibiotic that suppresses bacterial contamination.
  • Agar: 15.5 g – A solidifying agent that provides a firm surface for fungal growth.

Preparation:

  1. Dissolve: Mix the ingredients in 1 liter of distilled water.
  2. Sterilize: Autoclave at 121°C for 15 minutes.
  3. Pour: Cool the medium to about 45-50°C and pour into sterile Petri dishes.

Test Procedure

  1. Inoculate the sample onto the surface of the agar.
  2. Use a sterile loop or swab for even spreading.
  3. Incubate the plates at 25-30°C.
  4. Observe the plates regularly for fungal growth.
  5. Note the color, shape, and texture of fungal colonies.
  6. Identify fungi based on colony morphology and pigmentation.
  7. Compare growth with control plates if needed.
  8. Document findings and interpret results based on fungal characteristics.
  9. Dispose of used plates following biohazard protocols.

Colony Characteristics

  1. Fungal colonies appear distinct and colorful.
  2. Colonies often show shades of pink or red.
  3. Colony edges are usually well-defined and sharp.
  4. Growth stays localized due to rose bengal dye.
  5. Colonies can be round or irregular in shape.
  6. The texture varies from smooth to velvety or powdery.
  7. Some fungi produce aerial mycelium on the surface.
  8. Colony size ranges from small to medium.
  9. Color intensity may vary with fungal species.
  10. Yeasts form moist, creamy colonies.
  11. Molds generally form dry, filamentous colonies.
  12. Pigmentation helps differentiate between fungal species.
  13. Growth may be inhibited by chloramphenicol.
  14. Colony elevation can be flat, raised, or convex.
  15. Fungi exhibit rapid or slow growth rates depending on the species.

Keynotes

Rose Bengal Agar (RBA) is a selective medium for fungal isolation. It contains rose bengal dye, which inhibits bacterial growth and restricts fungal colony spread. Chloramphenicol, an antibiotic, further suppresses bacterial contamination. RBA supports a wide range of fungi, including yeasts and molds. Colonies often appear pink or red due to the dye. The medium enhances fungal differentiation based on colony color and morphology. Fungal colonies are distinct, with well-defined edges. Textures vary from smooth to velvety or powdery. Yeasts form moist, creamy colonies, while molds form dry, filamentous colonies. The medium’s nutrients promote fungal growth, but not bacteria. RBA is useful in clinical and environmental mycology. It helps identify fungi based on distinct colony characteristics. Regular observation of plates is crucial for accurate identification. Overall, RBA is a reliable tool for fungal isolation and differentiation.

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