Modified Letheen Agar Base is used for the determination of bacterial activity of quaternary ammonium compounds.

Approximate Formula:

Ingredients	gm/liter
Peptone from Meat	10.0
Peptone from Casein	10.0
Meat Extract	3.0
Yeast Extract	2.0
Sodium Chlorid	5.0
D(+) Glucose	1.0
Lecithin	1.0
Sodium Bisulfite	0.1
Agar	20.0
Final pH7.2±0.2 at 25℃

*Adjusted and/or supplemented as required to meet performance criteria.

Directions: 

Suspend 52.1g of the medium and 7 ml of Tween-80 in one liter of distilled water or deionized water. Mix well. Heat and boil for 1 minute with frequent agitation to completely dissolve the powder. Sterilize in an autoclave at 121℃(15 lbs.) for 15 minutes. Pour into petri dishes.

Principle and Interpretation: 

Peptone, meat extract and yeast extract provide nitrogen source for the growth of microorganisms, vitamin and growth factor. Sodium chloride provides a suitable osmotic environment. Glucose is a fermentable sugar. Lecithin can neutralize quaternary ammonium salt, Tween-80 can neutralize phenol, hexachlorophenol, formalin, the combination of which can neutralize ethanol. Sodium bisulfite partially neutralize the preservative systems commonly found in cosmetics. Agar is the solidifying agent of the medium.

Appearance:

Dehydrated medium is a free-flowing yellowish granule. The prepared medium is a kind of yellowish transparent gel.

Precautions: 

This medium is for laboratory use only. Dried medium which is past shelf life, caking or color variation cannot be used.

Storage conditions and Shelf life:

Modified Letheen Agar Base must be stored tightly capped in the original container at 5-30°C. The dehydrated medium has a shelf life of 3 years from date of manufacturing. Prepared medium may be stored, out of direct light at 2-8°C.

Quality control: 

Prepare the culture medium as per label directions. Inoculate and incubate at 36±1℃ for 24 hours.

Microorganism	Strains Number	Inoculum (CFU)	Growth	Recovery
Staphylococcus aureus	ATCC 6538	10-100	Luxuriant	≥70%
Pseudomonas aeruginosa	ATCC 9027	10-100	Luxuriant	≥70%
Escherichia coli	ATCC 25922	10-100	Luxuriant	≥70%
Salmonella typhimurium	ATCC 14028	10-100	Luxuriant	≥70%

References:
1. FDA Bacteriological Analytical Manual (BAM), 8th ed. (1995), chapter 23:Microbiological Methods for Cosmetics, Letheen Agar (modified) = M 78,etheen Broth (modified) = M 79.