604 Modern Food Microbiology
Purified LLO has been shown to share in common with SLO and PLO the following properties:
activated by SH-compounds such as cysteine, inhibited by low quantities of cholesterol, and common antigenic sites as evidenced by immunological cross-reactivity. Unlike SLO, LLO is active at
a pH of 5.5 but not at pH 7.0, suggesting the possibility of its activity in macrophage phagosomes
(phagolysosomes). Its LD 50 for mice is about 0.8 µg, and it induces an inflammatory response when
injected intradermally.
44 It appears that LLO and the other poreforming toxins evolved from a single
progenitor gene.
Intracellular Invasion
When L. monocytogenes is contracted via the oral route, it apparently colonizes the intestinal tract
by mechanisms that are poorly understood. From the intestinal tract, the organism invades tissues,
including the placenta in pregnant women, and enters the blood stream, from which it reaches other
susceptible body cells. As an intracellular pathogen, it must first enter susceptible cells, and then it
must possess means of replicating within these cells. In the case of phagocytes, entry occurs in two
steps: directly into phagosomes and from the phagosomes into the phagocyte’s cytoplasm.
Entry or uptake into nonphagocytic cells is different. In nonphagocytic cell lines, uptake requires
surface-bound proteins of the bacterium designated In1A and In1B.
78 The former has a molecular
weight of 88 kDa and the latter 65 kDa. They are involved in aiding the entry of L. monocytogenes cells into host cells. The In1A protein, i.e., internalin, and its mammalian surface receptor is
E-cadherin. It is required for entry into cultured epithelial cells, whereas In1B is required for invasion
of cultured mouse hepatocytes.
32 Another invasion-associated protein of Listeria is p60, a 60-kDa
protein encoded by the iap gene. It is secreted by all species of Listeria. Another surface protein, ActA
(90 kDa), is required for actin polymerization and it allows for intracytoplasmic movement of cells.
61
Ami (ca. 90 kDa) is located on the surface of L. monocytogenes and it is a bacteriolysin. From a study
of 150 food isolates, Ami was present in 149 while 283 of 300 human isolates contained this protein.
61
All of the positive strains contained LLO, InlB, and ActA.
L. monocytogenes survives inside macrophages by escaping from phagolysosomal membranes into
the cytoplasm (cytosol), and this process is facilitated in part by LLO. Once inside the cytosol, the surface protein ActA (encoded by actA) aids in the formation of actin tails that propel the organism toward
the cytoplasmic membrane. At the membrane, double membrane vacuoles form. With LLO and the two
bacterial phospholipases, the phosphatidylinositol-specific phospholipase C (encoded by plcA) and the
broad-range phospholipase C (encoded by plcB), the bacteria are freed and the process is repeated upon
entry of bacteria into adjacent host cells. The latter occurs following the pushing out of the membrane
to form a filopodium (a projection), which is absorbed by an adjacent cell and the invasion process is
repeated. Thus, the spread of L. monocytogenes from cell to cell occurs without the bacterium having
to leave the inner parts of host cells. For more information, see reference 97 and Chapter 22.
Monocytosis-Producing Activity
An interesting yet incompletely understood part of the L. monocytogenes cell is a lipid-containing
component of the cell envelope that shares at least one property with the lipopolysaccharide (LPS) that
is typical of Gram-negative bacteria. In Gram-negative bacteria, LPS is located in the outer membrane,
but listeriae and other Gram-positive bacteria do not possess outer membranes. The L. monocytogenes
substance is lipoteichoic acid (LTA). It was shown several decades ago that phenol–water extracts of
L. monocytogenes cells induce the production of monocytes, and it was because of this monocytosisproducing activity (MPA) factor that the organism was given the species name monocytogenes. This
Purified LLO has been shown to share in common with SLO and PLO the following properties:
activated by SH-compounds such as cysteine, inhibited by low quantities of cholesterol, and common antigenic sites as evidenced by immunological cross-reactivity. Unlike SLO, LLO is active at
a pH of 5.5 but not at pH 7.0, suggesting the possibility of its activity in macrophage phagosomes
(phagolysosomes). Its LD 50 for mice is about 0.8 µg, and it induces an inflammatory response when
injected intradermally.
44 It appears that LLO and the other poreforming toxins evolved from a single
progenitor gene.
Intracellular Invasion
When L. monocytogenes is contracted via the oral route, it apparently colonizes the intestinal tract
by mechanisms that are poorly understood. From the intestinal tract, the organism invades tissues,
including the placenta in pregnant women, and enters the blood stream, from which it reaches other
susceptible body cells. As an intracellular pathogen, it must first enter susceptible cells, and then it
must possess means of replicating within these cells. In the case of phagocytes, entry occurs in two
steps: directly into phagosomes and from the phagosomes into the phagocyte’s cytoplasm.
Entry or uptake into nonphagocytic cells is different. In nonphagocytic cell lines, uptake requires
surface-bound proteins of the bacterium designated In1A and In1B.
78 The former has a molecular
weight of 88 kDa and the latter 65 kDa. They are involved in aiding the entry of L. monocytogenes cells into host cells. The In1A protein, i.e., internalin, and its mammalian surface receptor is
E-cadherin. It is required for entry into cultured epithelial cells, whereas In1B is required for invasion
of cultured mouse hepatocytes.
32 Another invasion-associated protein of Listeria is p60, a 60-kDa
protein encoded by the iap gene. It is secreted by all species of Listeria. Another surface protein, ActA
(90 kDa), is required for actin polymerization and it allows for intracytoplasmic movement of cells.
61
Ami (ca. 90 kDa) is located on the surface of L. monocytogenes and it is a bacteriolysin. From a study
of 150 food isolates, Ami was present in 149 while 283 of 300 human isolates contained this protein.
61
All of the positive strains contained LLO, InlB, and ActA.
L. monocytogenes survives inside macrophages by escaping from phagolysosomal membranes into
the cytoplasm (cytosol), and this process is facilitated in part by LLO. Once inside the cytosol, the surface protein ActA (encoded by actA) aids in the formation of actin tails that propel the organism toward
the cytoplasmic membrane. At the membrane, double membrane vacuoles form. With LLO and the two
bacterial phospholipases, the phosphatidylinositol-specific phospholipase C (encoded by plcA) and the
broad-range phospholipase C (encoded by plcB), the bacteria are freed and the process is repeated upon
entry of bacteria into adjacent host cells. The latter occurs following the pushing out of the membrane
to form a filopodium (a projection), which is absorbed by an adjacent cell and the invasion process is
repeated. Thus, the spread of L. monocytogenes from cell to cell occurs without the bacterium having
to leave the inner parts of host cells. For more information, see reference 97 and Chapter 22.
Monocytosis-Producing Activity
An interesting yet incompletely understood part of the L. monocytogenes cell is a lipid-containing
component of the cell envelope that shares at least one property with the lipopolysaccharide (LPS) that
is typical of Gram-negative bacteria. In Gram-negative bacteria, LPS is located in the outer membrane,
but listeriae and other Gram-positive bacteria do not possess outer membranes. The L. monocytogenes
substance is lipoteichoic acid (LTA). It was shown several decades ago that phenol–water extracts of
L. monocytogenes cells induce the production of monocytes, and it was because of this monocytosisproducing activity (MPA) factor that the organism was given the species name monocytogenes. This