Letheen Agar

Intended Use

Letheen Agar is used to inactivate quarternary ammonium compounds and other preservatives when determining the number of bacteria present in cosmetics and other materials.

Summary and Explanation

The value of a highly nutritional solid medium containing neutralizing agents for quaternary ammonium compounds in sanitizers was described by Weber and Black1 in 1948. The
addition of lecithin and polysorbate 80 (Tween™* 80) to Tryptone Glucose Extract (TGE) agar resulted in a medium that effectively neutralizes quaternary ammonium compounds in the testing of germicidal activity. Letheen Agar is a modification of TGE agar with the addition of lecithin and polysorbate 80.

Letheen Agar is specified for use by the American Society for Testing and Materials (ASTM) in the Standard Test Method for Preservatives in Water-Containing Cosmetics.4

*Tween is a trademark of ICI Americas, Inc.

Principles of the Procedure

Letheen Agar contains beef extract and peptone which provide the carbon and nitrogen sources required for growth of a wide variety of organisms. Dextrose is provided as a source of fermentable carbohydrate. Agar is the solidifying agent. Lecithin and polysorbate 80 are added to neutralize surface disinfectants.2,5,6 Lecithin is added to neutralize quaternary
ammonium compounds and polysorbate 80 is incorporated to neutralize phenols, hexachlorophene, formalin and, with lecithin, ethanol.7

User Quality Control

Identity Specifications
Letheen Agar
Dehydrated Appearance: Tan, moist appearance, with a few clumps.
Solution:                        3.2% solution, soluble in purified water upon
                                    boiling. Solution is light to medium amber,
                                    clear to slightly opalescent, may have a slight,
                                    fine precipitate (opalescent immediately after autoclaving).
Prepared Appearance:     Light to medium amber, slightly opalescent, may
                                    have a slight precipitate.
Reaction of 3.2%
Solution at 25°C:            pH 7.0 ± 0.2

Cultural Response
Letheen Agar
Prepare the medium per label directions. Inoculate and incubate at 35 ± 2°C for 40-48 hours.

Formula

Letheen Agar
Approximate Formula* Per Liter
Beef Extract.................................................................. 3.0 g
Pancreatic Digest of Casein............................................. 5.0 g
Dextrose...................................................................... 1.0 g
Agar........................................................................... 15.0 g
Polysorbate 80.............................................................. 7.0 g
Lecithin........................................................................ 1.0 g

Directions for Preparation from Dehydrated Product

1. Suspend the powder in 1 L of purified water:
    Letheen Agar – 32 g; Mix thoroughly.
2. Heat with frequent agitation and boil for 1 minute to completely dissolve the powder.
3. Autoclave at 121°C for 15 minutes.
4. Test samples of the finished product for performance using stable, typical control cultures.
NOTE: The dehydrated Letheen Agar has a characteristic “brown sugar” appearance and may seem moist. This does not indicate deterioration.

Procedure

Letheen Agar is used in a variety of procedures. Consult appropriate references for further information.3,4

Expected Results

Refer to appropriate references and procedures for results.3,4
*Store at 2-8°C.

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