Etiological agent = Listeria monocytogenes
(a gram [+] rod)
Listeria monocytogenes is a facultative intracellular
organism that is able to escape from the phagolysosome and replicate
in the cytoplasm of phagocytic cells. Escape from the phagolysosome
is aided by expression of listeriolysin O toxin by the bacteria.
Ecology of Listeria:
Listeria monocytogenes is very widely distributed
in the environment. It can survive for months to years in the
soil.
Listeria monocytogenes is shed in the feces of both
clinically ill and subclinically infected animals, particularly
ruminants. This is the major source of contamination of the environment,
but direct zoonotic transmission between infected animals and
humans is relatively uncommon.
Listeria monocytogenes cannot replicate at pH<5.
One implication of this is that the organism does not replicate
in properly prepared silage. However, poorly fermented silage
with pH>5 can be an important source of the organism on the
farm.
Listeria monocytogenes is also shed by subclinically-infected
humans.
As many as 10-30% of people may be shedding the organism
at any given time!
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