Letheen Agar, Modified, Granulated (Modified Letheen Agar, Granulated)

Principle And Interpretation

In the early 40s, Weber and Black recommended the use of lecithin and polysorbates to neutralize the antimicrobial action of the quaternary ammonium compounds (3). In 1965, the methodology was accepted by AOAC for the antimicrobial assays and extended their use to all the cationic detergents. In 1978, the FDA incorporated it as pre-enrichment medium for every microbial examination of cosmetics. Letheen Agar, Modified is used to partially inactivate the preservatives in cosmetics being analyzed for the microbial content (1). This medium was originally recommended by APHA for use in microbial testing of water (2). Peptone, tryptone, HM Peptone B and yeast extract provide nitrogenous and carbonaceous compounds, long chain amino acids, vitamins and trace elements to the microorganisms. Incorporation of lecithin and polysorbate 80 to the medium enables the recovery of bacteria from materials containing residues of disinfectant compounds or preservatives used in cosmetics. Polysorbate 80 is added to nullify phenolic compounds, hexachlorophene, formalin and along with lecithin neutralizes ethyl alcohol (4). Lecithin also neutralizes quaternary ammonium compounds present in the cosmetics. Sodium chloride maintains the osmotic balance of the medium.