Listeria species

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Overview

• 6 species
• Saprophytes in soil
• L. monocytogenes and L. ivanovii are pathogens
• Carried by sheep and goats and shed in faeces and milk especially during stress
• Can cause septicaemia, encephalitis and abortion
• Outbreaks of listeriosis often linked to silage feeding

Characteristics

• Small Gram negative rods
• Catalase positive, oxidase negative
• Motile
• Facultative anaerobes
• L. monocytogenes is haemolytic on blood agar due to a cytolytic protein, listeriolysin; grows at range of pH values and temperatures
• L. ivanovii produces strong haemolytic zone
• Small, smooth, transparent colonies after 24 hours incubation
• Grow on non-enriched media

Pathogenesis and pathogenicity

• Infection by ingestion of contaminated feed
• Bacteria penetrate M cells in intestinal Peyer's patches
• Spread to tissues via blood and lymph
• Transplacental transmission in pregnant animals
• Bacteria may gain entry via breaks in oral or nasal mucosa, migrate in cranial nerves to cause neural signs
• Causes formation of microabscesses and perivascular lymphocytic cuffs in brainstem
• L. monocytogenes can replicate within phagocytic and non-phagocytic cell, and pass between cells without being exposed to the immune system
• Surface proteins known as internalins allow adherence and uptake of the bacteria into cells
• Listeriolysin produced by virulent strains destroys membranes of phagocytic vacuoles, releasing the bacteria into the cytoplasm
• Listeria are motile in the cytoplasm
• Bacteria induce formation of pseudopod projections in the cytoplasmic membrane, which are taken up with the bacteria into adjacent cells