Introduction
Dorset egg medium is a solid egg-based medium. Microbiologists widely use it to cultivate, maintain, and store certain pathogenic bacteria, especially Mycobacterium species. They also use this medium as a stock culture medium because it preserves bacterial viability for long periods without significant change.
Composition of Dorset Egg Medium
Whole eggs (fresh, sterile) → Provide nutrients and solidifying base
Saline or buffer → Maintains osmotic balance
Glycerol → Enhances growth of Mycobacterium tuberculosis (not used for M. bovis)
Malachite green (sometimes) → Inhibits contaminants
pH: ~7.2–7.4
Principle of Dorset Egg Medium
The egg proteins coagulate on heating, forming a solid medium.
Eggs provide a rich nutrient source for fastidious bacteria.
Glycerol selectively enhances growth of Mycobacterium tuberculosis.
When malachite green is added, it prevents growth of contaminants, allowing selective isolati
Preparation of Dorset Egg Medium
Collect fresh hen’s eggs → wash thoroughly, disinfect shell with 70% alcohol.
Break eggs aseptically, beat well to form a homogenous mixture.
Add 50% saline and glycerol (optional).
Dispense into sterile tubes.
Slant the tubes to form slopes.
Coagulate by inspissation (heating at 85–90°C for 30–45 minutes on 3 consecutive days).
Store at 2–8°C.
Sterilization
Achieved by inspissation (not autoclaving, since eggs coagulate).
Temperature: 85–90°C for 30–45 min for 3 days.
Uses of Dorset Egg Medium
Cultivation of Mycobacterium tuberculosis
Especially M. tuberculosis (grows better with glycerol).
Storage of bacterial cultures
Many pathogens can be preserved for long periods.
Subculture medium
Used to maintain stock strains in microbiology laboratories.
Selective isolation
When supplemented with malachite green, it suppresses contaminants and enhances mycobacterial growth.
Advantages
Simple, inexpensive, and easy to prepare.
Excellent for long-term preservation of cultures.
Provides a nutrient-rich environment for fastidious organisms.
Limitations
Not as selective as Lowenstein-Jensen (LJ) medium.
Requires careful handling of eggs to avoid contamination.
Not suitable for rapid growth organisms.
Conclusion
Dorset Egg Medium is an important egg-based medium used in microbiology for cultivation and preservation of
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