Introduction of Dorset Egg Medium
In 1903, Dorset inaugurated the first egg-based medium, and later it was further modified by Löwenstein-Jensen and Ogawa using whole eggs as nutrients and malachite green to inhibit the growth of contaminants. Dorset Egg Medium is used for culturing mycobacteria and it is a non-selective medium well suited for the maintenance of pure cultures of Mycobacterium. The medium is also used for the maintenance and transport of Neisseria meningitidis and Haemophilus influenzae type b.
Principle of Dorset Egg Medium
Glycerol and eggs both supply the fatty acids and the proteins required by the metabolism of microbes (mycobacteria). The coagulation of egg albumin during the sterilization makes a solid medium for inoculum.
Composition of Dorset Egg Medium
Homogenized whole egg: 95.0 mL
Glycerol: 5.0 ml
Final pH=7.0 ± 0.2
Preparation of Dorset Egg Medium
- Prepare 95 ml of a uniform suspension of fresh eggs under aseptic conditions. Avoid whipping air into suspension during the collection and mixing.
- Aseptically mix the 95 ml of egg suspension with 5 ml of the sterile glycerol, avoiding air bubbles.
- Dispense the prepared medium into sterile screw-cap test tubes.
- Put the tubes in a slanted position and heat at 85°C for 45 minutes.
Storage and Shelf Life
Store in a tightly-closed screw-cap test tubes and the prepared medium at 2-8°C, preferably in sealed plastic bags to prevent loss of moisture and contamination. The shelf life of this prepared medium is up to 1 year. Use before the expiry date on the label.
The procedure of Dorset Egg Medium
- Inoculate the medium with the microbes.
- Incubate the tubes at 35-37°C for 2 weeks, in an atmosphere containing 10% CO2.
- Examine the growth on the tubes.
Colony Morphology of Dorset Egg Medium
Microbes –Growth–Colony Morphology
Mycobacterium fortuitum-Luxuriant-Smooth hemispheric colonies, usually off white or cream colored and maybe butyrous, waxy, multilobate, and even rosette clustered (fast grower-within 2–4 days)
Mycobacterium intracellulare-Luxuriant- smooth transparent, opaque, and rough colony
Uses of Dorset Egg Medium
- It Is a non-selective medium-well suited for the maintenance of a pure culture of Mycobacterium species.
- The medium is also applied for maintenance and transport of Neisseria meningitidis and Haemophilus influenzae serotype b.
Keynotes on Dorset Egg Medium
- Acid Egg Medium Slant with pyruvate is a modified type of Dorset Egg Medium that acts as a selective medium for M. tuberculosis.
- Ogawa medium differs from Lowenstein-Jensen medium due to lacking glycerol, magnesium citrate, and magnesium sulfate.
- Mycobacterium fortuitum grows on MacConkey agar
- Middlebrook 7H10 is both liquid and solid forms of media for mycobacteria cultivation and antituberculous drugs testing.
- Dorset egg medium in the different compositions is also used as an enriched medium for the cultivation of Corynebacterium diphtheriae for volutin granules expression.