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General characteristics of Listeria
- Morphology - slender, gram-positive rods that do not form spores
- Avid intracellular parasites that reside in the cytoplasm of infected cells.
- Destinguished from Corynebacterium and Streptococcus because it exhibits tumbling motility and is catalase-positive
- Asymmetric actin polymerization as motility
- Salt tolerant and can survive and grow at temps below 1°C
Epidemiology of Listeria
- Listeria monocytogenes - only species affecting humans
- Other Listeria species widespread in nature
- Causes Listerosis - sproadic cases or small epidemics, typically foodborne.
- - processed dairy, ground meats, poultry, deli meats.
- Grows at 4°C - presents issue for food storage in refrigerators and factories.
- Infections most common in pregnant women, fetuses and newborns, the elderly and immunocompromised individuals.
Pathogenicity of Listeria
- Facultative intracellular parasite,
- Attaches and enters cells by normal phagocytic mechanisms.
- Escapes phagocytic vacuole using listeriolysin O.
- -mutants lacking this hemolysin are avirulent.
- Low pathogenicity (infects mostly immunocompromised and is not transmitted via direct contact), but high virulence (once infected, it is very hard to get rid of and has a high incidence of disease).
Transmission of Listeria from one cell to another
- After escaping the phagolysosome with the use of listeriolysin O the bacteria multiplies inside the cytoplasm of the macrophage.
- The bacteria polymerize the actin fibers of the host cell in order to propel itself across the cell and into the neighboring cell.Phospholipase C (hemolysin) is used to penetrate the double membrane vacuole once inside a new neighboring cell, thus avoiding the cells of our immune system
Clinical significance of L. monocytogenes
- Septicemia and meningitis are most commonly reported listeriosis.
- Pregnant women may exhibit flu like symptoms and it can be passed to the baby both systemically and via asymptomatic vaginal infection.
- - Common cause of newborn meningitis and spontaneous abortion.
Laboratory Identification of L. monocytogenes
- May be isolated from blood, CSF and other specimens
- On Blood Agar - produces small colonies surrounded by narrow β-hemolysis.
- Motile, catalase positive, gram positive rod
Treatment and prevention of L. monocytogenes
- A variety of antibiotics are effective.
- - Ampicillin, trimethoprim/sulfamethoxazole
Prevention accomplished by proper food handling and prep procedures.