Standard Methods Agar with Tween 80 and Lecithin is formulated as per APHA (1) for the enumeration of microorganisms from flat and nonporous surfaces. For the purpose of this medium the plates should be prepared carefully to ensure the presence of meniscus of agar extending above the top of the poured plate.
Casein enzymic hydrolysate provides amino acids while yeast extract supplies vitamin B complex and dextrose gives energy to microorganisms. Polysorbate 80 and lecithin act as neutralizers to inactivate the residual disinfectants where the samples are collected (2, 3). Lecithin inactivates quaternary ammonium compounds whereas polysorbate 80 neutralizes formalin, phenolic disinfectants, hexachlorophene etc.
Directions: Suspend 29.2 grams in 1000 ml distilled water. Heat to boiling to dissolve the medium completely. Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes. Mix well and pour into sterile Petri plates.