Listeria species
Jump to navigation
Jump to search
 |
This article has been peer reviewed but is awaiting expert review. If you would like to help with this, please see more information about expert reviewing. |
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
Clinical infections
- L. ivanovii causes sporadic abortion in sheep and cattle
- L. innocua rarely causes ovine meningoencephalitis
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