Difference between revisions of "Category:Listeria species"

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, abortion and endophthalmitis in ruminants
• Outbreaks of listeriosis often linked to silage feeding
• Occurs in North and East Europe and North America

Characteristics

• Small Gram positive rods
• Catalase positive, oxidase negative
• Tumbling motility
• Facultative anaerobes
• Intracellular pathogens
• 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
• Cell-mediated immune response required for protection

Diagnosis

• Specimens should include CSF in neural cases, cotyledons in abortion, liver, spleen and blood in septicaemia
• Immunofluorescence using monoclonal antibodies
• Histology of brain demonstrates microabscesses and lymphocytic cuffing in brainstem
• Smears of cotyledons
• High protein and cell counts in CSF
• Isolation on blood and MacConkey agar

Treatment and control

• Ampicillin or amoxycillin in early stages of septicaemic listeriosis
• Sub-conjuntival antibiotics and corticosteroids for ocular listeriosis
• Avoid poor quality silage and discontinue silage-feeding in an outbreak

Causes abortions in cattle and sheep last third of pregnancy. Foetal membranes often retained leading to metritis. The placenta is covered by purulent exudate and there are focal pinpoint yellow lesions in foetal liver.