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First described in 1926 by Murray, Webb, and Swann,¹ Listeria monocytogenes is recognized as a problem of importance in public health, especially by the food and dairy industries. L. monocytogenes has been documented as the cause of food-associated illness in the Canadian Maritime Provinces, Boston and southern California. These outbreaks of listeriosis were notable because of the high casefatality rate.²

The isolation and identification of L. monocytogenes has been difficult because of the need for a faster enrichment procedure and the lack of a suitable selective and differential medium for isolating the organism from suspected sources in mixed populations. Many common food contaminants such as the streptococci, Bacillus species, Eschericia coli, Pseudomonas aeruginosa and Proteus vulgaris interfere with the isolation of Listeria monocytogenes.³

The Listeria are short, gram-positive, non-sporeforming, motile rods. The organism can grow over a wide temperature range. It is characterized by its ability to hydrolyze esculin and by tolerance of salt, tumbling motility and beta hemolysis.

Presumptive identification of Listeria is based on the esculin hydrolysis reaction which can be seen as a darkening of the medium after incubation in Fraser Broth⁴ or on Oxford Medium Base.⁵ Motility can be observed as an umbrella-type growth in Motility Test Medium. A narrow zone of beta-hemolysls around and under Listeria colonies can be observed on blood agar (McBride Listeria Agar or Columbia Blood Agar Base).⁶

After successful isolation of the organism, final identification of Listeria is based on biochemical characterization and serological confirmation.

Identity Specifications

OXFORD MEDIUM BASE

Dehydrated medium: tan, homogeneous, free-flowing

Reaction of 5.75% solution: pH 7.2 ± 0.2 at 25°C

Prepared plates: light to medium amber, slightly opalescent

Typical Cultural Response After 24 - 40 H at 35°C

The cultures listed above are the minimum that should be used for performance testing.

For Laboratory Use.

WARNING—OXFORD MEDIUM BASE CAUSES IRRITATION. Avoid contact with eyes, skin and clothing. Wash thoroughly after handling.

FIRST AID: IN CASE OF CONTACT, immediately flush eyes with plenty of water for at least 15 min. Call a physician. Flush skin with water. Wash clothing before reuse.

WARNING—OXFORD ANTIMICROBIC SUPPLEMENT IS HARMFUL IF SWALLOWED OR INHALED. MAY CAUSE IRRITATION. MAY CAUSE BIRTH DEFECTS. Avoid contact with eyes, skin and clothing. Do not breathe dust.

FIRST AID: IF SWALLOWED, induce vomiting. Call a physician. Never give anything by mouth to an unconscious person. IF INHALED, remove to fresh air. If not breathing, give artificial respiration, preferably mouth-to-mouth. If breathing is difficult, give oxygen. Call a physician. IN CASE OF CONTACT, immediately flush eyes with plenty of water for al least 15 min. Call a physician. Flush skin with water.

Follow proper, established laboratory procedures in handling and disposing of infectious materials.