LPM Agar Base

Intended Use

LPM Agar Base is used with Listeria Selective Supplement for isolating and cultivating Listeria monocytogenes.

Summary and Explanation

First described in 1926 by Murray, Webb and Swann,1 Listeria monocytogenes is a widespread problem in public health and the food industries. This organism can cause human illness and death, particularly in immunocompromised individuals and pregnant women.2 The first reported foodborne outbreak of listeriosis was in 1985,3 and since then, microbiological and epidemiological evidence from both sporadic and epidemic cases of listeriosis has shown that the principal route of transmission is via the consumption of foodstuffs contaminated with L. monocytogenes.4
Implicated vehicles of transmission include turkey frankfurters,5 coleslaw, pasteurized milk, Mexican-style cheese, paté, and pickled pork tongue. The organism has been isolated from
commercial dairy and other food processing plants, and is ubiquitous in nature, being present in a wide range of unprocessed foods and in soil, sewage, silage and river water.6

Listeria species grow over a pH range of 4.4-9.6, and survive in food products with pH levels outside these parameters.7 Listeria spp. are microaerophilic, gram-positive, asporogenous,
non-encapsulated, non-branching, regular, short, motile rods. Motility is most pronounced at 20°C. 

The most common contaminating bacteria found in food sources potentially containing Listeria are: streptococci, especially the enterococci, micrococci and Bacillus species, Escherichia coli, Pseudomonas aeruginosa and Proteus vulgaris.8 

Identification of Listeria is based on successful isolation of the organism, biochemical characterization and serological confirmation.
LPM Agar, a modification of McBride Listeria Agar, was developed by Lee and McClain9 to recover low numbers of L. monocytogenes from samples with profusely mixed microflora. Its use is recommended when testing food and dairy samples and clinical specimens for Listeria.

User Quality Control

Identity Specifications
LPM Agar Base
Dehydrated Appearance: Light tan, homogeneous, may have a tendency to clump.
Solution:                       5.05% solution, soluble in purified water upon
                                   boiling. Solution is light to medium amber, very
                                   slightly to slightly opalescent.
Prepared Appearance:    Light to medium amber, slightly opalescent.
Reaction of 5.05%
Solution at 25°C:          pH 7.3 ± 0.2
Listeria Selective Supplement
Lyophilized Appearance: White to pale yellow and dry.
Rehydrated Appearance: Clear to pale yellow to light tan yellow; clear.
Cultural Response
LPM Agar Base
Prepare the medium per label directions with Listeria Selective Supplement. Inoculate and incubate at 35 ± 2°C for 18-48 hours.

Principles of the Procedure

In LPM Agar, peptones and beef extract provide nitrogen, vitamins and minerals. Sodium chloride maintains the osmotic balance of the medium. Glycine anhydride is used for improved
recovery of Listeria. Lithium chloride, in an increased concentration, and phenylethanol are incorporated to aid in suppression of both gram-positive and gram-negative contaminants. Agar
is the solidifying agent. Listeria Selective Supplement is added to LPM Agar Base after autoclaving to inhibit staphylococci, bacilli and Proteus species.

Formula

LPM Agar Base
Approximate Formula* Per Liter
Pancreatic Digest of Casein.......................................... 5.0 g
Proteose Peptone No. 3............................................... 5.0 g
Beef Extract................................................................ 3.0 g
Sodium Chloride......................................................... 5.0 g
Lithium Chloride......................................................... 5.0 g
Glycine Anhydride..................................................... 10.0 g
Phenylethanol............................................................ 2.5 g
Agar........................................................................ 15.0 g
Listeria Selective Supplement
Formula Per 1 mL Vial
Moxalactam................................................................. 0.01 g
*Adjusted and/or supplemented as required to meet performance criteria.

Directions for Preparation from Dehydrated Product

LPM Agar Base
1. Suspend 50.5 g of the powder in 1 L of purified water. 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. Cool medium to 45-50°C.
5. Aseptically add 2.0 mL of the reconstituted Listeria Selective Supplement to 1 L of medium. Mix well.
6. Test samples of the finished product for performance using stable, typical control cultures.
Listeria Selective Supplement
1. Reconstitute each lyophilized vial by aseptically adding 2.0 mL of sterile purified water with a sterile syringe and needle.
2. Invert the vial several times to assure complete solution.

Procedure

Clinical specimens obtained from nonsterile sites should be selectively enriched for Listeria spp. before being plated. Refer to appropriate references for the procedure to use with clinical
specimens.10 For a procedure for isolating Listeria from milk, milk products and food samples, refer to an appropriate reference.7,11,12

Expected Results

Observe colonies under oblique transmitted light. Listeria colonies display a gray to blue color with a ground glass appearance.

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