Listeria monocytogenes (page 2)
(This chapter has 3 pages)
© Kenneth Todar, PhD
Pathogenesis
Listeria monocytogenes is presumably ingested with raw,
contaminated
food. An invasin secreted by the pathogenic bacteria enables the
listeriae
to penetrate host cells of the epithelial lining. The bacterium is
widely
distributed so this event may occur frequently. Normally, the immune
system
eliminates the infection before it spreads. Adults with no history of
listeriosis
have T lymphocytes primed specifically by Listeria antigens.
However,
if the immune system is compromised, systemic disease may develop. Listeria
monocytogenes multiplies not only extracellularly but also
intracellularly,
within macrophages after phagocytosis, or within parenchymal cells
which
are entered by induced phagocytosis.
In mice infected with L. monocytogenes, the bacteria first
appear
in macrophages and then spread to hepatocytes in the liver. The
bacteria
stimulate a CMI response that includes the production of TNF, gamma
interferon,
macrophage activating factors and a cytotoxic T cell response.
Possibly,
in humans, a failure to control L. monocytogenes by means of
CMI
allows the bacteria to spread systemically. As well, unlike other
bacterial
pathogens,
Listeria are able to penetrate the endothelial layer
of the placenta and thereby infect the fetus.
Virulence Factors
Growth at low temperatures
A peculiar property of L. monocytogenes that affects its
food-borne
transmission is the ability to multiply at low temperatures. The
bacteria
may therefore grow and accumulate in contaminated food stored in the
refrigerator.
So it is not surprising that listeriosis is usually associated with
ingestion
of milk, meat or vegetable products that have been held at
refrigeration
temperatures for a long period of time.
Growth and viability of
Listeria monocytogenes
in certain foods at freezing temperatures (-20oC and
refrigeration
temperatures (4oC) over a 12 week period. Adapted from Baron's (Online)
Medical Microbiology: Miscellaneous
Pathogenic Bacteria. 2008.
Motility
As in the case of Vibrio cholerae, wherein movement,
attachment
and penetration of the intestinal mucosa are determinants of infection
(if not disease), this was thought to be the situation with Listeria,
which
is also acquired by ingestion and must also find a way to attach to the
intestinal mucosa. With cholera, the actively-motile vibrios are
thought
to use their flagella to swim against the peristaltic movement of the
bowel
content and to penetrate (by swimming laterally) the mucosal lining of
the gut where they adhere. Curiously, although Listeria are
actively
motile by means of peritrichous flagella at room temperature
(20-25°C),
the organisms do not synthesize flagella at body temperatures
(37°C).
Instead, virulence is associated with another type of motility: the
ability
of the bacteria to move themselves into, within and between host cells
by polymerization of host cell actin at one end of the bacterium
("growing
actin tails") that can propel the bacteria through cytoplasm. However,
one should not totally dismiss the advantage of flagellar motility for
existence and spread of the bacteria outside of the immediate host
environment.
Listeria monocytogenes
Scanning EM showing Flagella
Adherence and invasion
Listeria can attach to and enter mammalian cells. The
bacterium
is thought to attach to epithelial cells of the GI tract by means of
D-galactose
residues on the bacterial surface which adhere to D-galactose receptors
on the host cells. If this is correct, it is the opposite of the way
that
most other bacterial pathogens are known to adhere, i.e., the bacterium
displays the protein or carbohydrate ligand on its surface and the host
displays the amino acid or sugar residue to which the ligand binds.
Having
said this, macrophages are well known to have "mannose binding
receptors"
on their surface whose function presumably is to ligand to bacterial
surface
polysaccharides that terminate in mannose, as a prelude to phagocytic
uptake.
The bacteria are then taken up by induced phagocytosis,
analogous
to the situation in Shigella. An 80 kDa membrane protein called
internalin
probably mediates invasion. A complement receptor on macrophages has
been
shown to be the internalin receptor, as well.
After engulfment, the bacterium may escape from the phagosome before
phagolysosome fusion occurs mediated by a toxin, which also acts as a
hemolysin,
listeriolysin
O (LLO). This toxin is one of the so-called SH-activated
hemolysins,
which are produced by a number of other Gram-positive bacteria, such as
group A streptococci (streptolysin O), pneumococci (pneumolysin), and Clostridium
perfringens. The hemolysin gene is located on the chromosome within
a cluster of other virulence genes that are all regulated by a common
promoter. Survival of the bacterium within the phagolysosome also
occurs,
aided
by the bacterium's ability to produce catalase and superoxide dismutase
which
neutralize
the effects of the phagocytic oxidative burst.
Additional genetic determinants are necessary for further steps in
the
intracellular life cycle of L. monocytogenes. One particular
gene
product, Act A (encoded by actA) promotes the
polymerization
of actin, a component of the host cell cytoskeleton, on the bacterial
surface.
Within the host cell environment, surrounded by a sheet of actin
filaments,
the bacteria reside and multiply. The growing actin sheet functions as
a propulsive force which drives the bacteria across the intracellular
pathways
until they finally reach the surface. Then, the host cell is induced to
form slim, long protrusions containing living L. monocytogenes.
Those cellular projections are engulfed by adjacent cells, including
non-professional
phagocytes such as parenchymal cells. By such a mechanism, direct
cell-to-cell
spread of Listeria in an infected tissue may occur without an
extracellular
stage.
Steps in the invasion of cells
and intracellular spread by L. monocytogenes. The bacterium
apparently
invades via the intestinal mucosa. It is thought to attach to
intestinal
cells by means of D-galactose residues on the bacterial surface which
adhere
to D-galactose receptors on susceptible intestinal cells The
bacterium
is taken up (including by non phagocytic cells) by induced
phagocytosis,
which is thought to be mediated by a membrane associated protein called
internalin. Once ingested the bacterium produces listeriolysin (LLO) to
escape from the phagosome. The bacterium then multiplies rapidly in the
cytoplasm and moves through the cytoplasm to invade adjacent cells by
polymerizing
actin to form long tails.
Other determinants of virulence
L. monocytogenes produces two other hemolysins besides LLO:
phosphatidylinositol-specific
phospholipase C (PI-PLC) and
phosphatidylcholine-specific
phospholipase C (PC-PLC). Unlike LLO, which lyses host cells
by forming a pore in the cell membrane, these phospholipases disrupt
membrane
lipids such as phosphatidylinositol and phosphatidylcholine (lecithin).
The bacterium also produces a Zn++ dependent protease
which
may act as some sort of exotoxin. Mutations in the encoding gene (mpl)
reduce virulence in the mouse model.
Finally, an operon called lmaBA encodes a 20 kDa
protein
located on the bacterial surface. The protein LMaA induces
delayed
type hypersensitivity and other CMI responses.
chapter continued
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