Foodborne Listeriosis
605
LTA fraction accounts for about 6% of the dry weight of cells and is associated with the plasma membrane. It has a molecular weight of about 1,000 Da, contains no amino acids or carbohydrates, and
stimulates only mononuclear cells.
42 It possesses low tissue toxicity and is serologically inactive
121 but
it kills macrophages in vitro.
42 It has been shown to share the following properties with LPS: it is pyrogenic and lethal in rabbits, produces a localized Schwartzman reaction, contains acylated hydroxy fatty
acids, produces a positive reaction with the Limulus amoebocyte lysate (LAL) reagent, contains 2-keto3-deoxyoctonic acid (KDO), and contains heptose. Regarding its LAL reactivity, 1 µg/ml was required
to produce a positive reaction
115 whereas the same can be achieved with picogram quantities of LPS.
Sphingomyelinase
L. ivanovii is known to be infectious for sheep, in which it causes abortions, and to be a prolific
producer of hemolysin on sheep erythrocytes. It has been shown to possess an LLO-like hemolysin
(ILO), sphingomyelinase, and lecithinase.
73 Sphingomyelinase has a molecular weight of 27,000
Da.
127 Whereas the LLO-like agent is responsible for the inner complete zone of hemolysis on sheep
erythrocytes, the halo of incomplete hemolysis that is enhanced by Rhodococcus equi appears to be
caused by the two enzymes noted. In one study, a mutant defective in sphingomyelinase and another
protein exhibited lower virulence than wild-type strains.
1
ANIMAL MODELS AND INFECTIOUS DOSE
The first animal model employed to test the virulence of L. monocytogenes was the administration
of a suspension of cells into the eye of a rabbit or guinea pig (Anton’s test), where 10
6 cells produced
conjunctivitis.
2 Chicken embryos have been studied by a large number of investigators. Inocula of
100 cells of L. monocytogenes into the allantoic sac of 10-day-old embryos led to death within 2–5
days, and the LD 50 was less than 6 × 10
2 cells for virulent strains. L. ivanovii is also lethal by this
method. Injections of 100–30,000 cells/egg into the chorioallantoic membrane of 10-day-old chick
embryos resulted in death within 72 hours compared to about 5 days for mice.
123 Although Anton’s
test and chick embryos may be used to assess the relative virulence of strains of listeriae, the mouse
is the model of choice for the additional information that it gives relative to cellular immunity.
Not only the mouse is the most widely used laboratory animal for virulence studies of listeriae,
but also it is widely used in studies of T cell immunity in general. This model is employed by use of
normal, baby, juvenile, or adult mice, as well as a variety of specially bred strains such as athymic
(T cell-deficient) nude mice. Listerial cells have been administered intraperitoneally (IP), intravenously
(IV), and intragastrically (IG). When normal adult mice are used, all smooth and hemolytic strains
of L. monocytogenes at levels of 10
3 –10
4 /mouse multiply in the spleen.
59 With many strains, inocula
of 10
5 –10
6 are lethal to normal adult mice, although numbers as high as 7 × 10
9 have been found
necessary to produce an LD 50 . A low of 50 cells for 15-g mice has been reported (see below).
Whereas the IP route of injection is often used for mice, IG administration is employed to assess
gastrointestinal behavior of listeriae. The administration of L. monocytogenes to 15-g mice by the
IG route produced more rapid infection and more deaths in the first 3 days of the 6-day test than
by IP.
105 By this method, the approximate 50% lethal dose (ALD 50 ) ranged from 50 to 4.4 × 10
5
cells for 15 food and clinical isolates of L. monocytogenes.
105 Six- to eight-week-old mice were given
IP and oral challenges of a serovar 4b strain by one group to study their effect under normal and
compromised states. Cells were suspended in 11% nonfat milk solids and administered to four groups
of mice: normal, hydrocortisone treated, pregnant, and cimetidine treated. Minimum numbers of cells
605
LTA fraction accounts for about 6% of the dry weight of cells and is associated with the plasma membrane. It has a molecular weight of about 1,000 Da, contains no amino acids or carbohydrates, and
stimulates only mononuclear cells.
42 It possesses low tissue toxicity and is serologically inactive
121 but
it kills macrophages in vitro.
42 It has been shown to share the following properties with LPS: it is pyrogenic and lethal in rabbits, produces a localized Schwartzman reaction, contains acylated hydroxy fatty
acids, produces a positive reaction with the Limulus amoebocyte lysate (LAL) reagent, contains 2-keto3-deoxyoctonic acid (KDO), and contains heptose. Regarding its LAL reactivity, 1 µg/ml was required
to produce a positive reaction
115 whereas the same can be achieved with picogram quantities of LPS.
Sphingomyelinase
L. ivanovii is known to be infectious for sheep, in which it causes abortions, and to be a prolific
producer of hemolysin on sheep erythrocytes. It has been shown to possess an LLO-like hemolysin
(ILO), sphingomyelinase, and lecithinase.
73 Sphingomyelinase has a molecular weight of 27,000
Da.
127 Whereas the LLO-like agent is responsible for the inner complete zone of hemolysis on sheep
erythrocytes, the halo of incomplete hemolysis that is enhanced by Rhodococcus equi appears to be
caused by the two enzymes noted. In one study, a mutant defective in sphingomyelinase and another
protein exhibited lower virulence than wild-type strains.
1
ANIMAL MODELS AND INFECTIOUS DOSE
The first animal model employed to test the virulence of L. monocytogenes was the administration
of a suspension of cells into the eye of a rabbit or guinea pig (Anton’s test), where 10
6 cells produced
conjunctivitis.
2 Chicken embryos have been studied by a large number of investigators. Inocula of
100 cells of L. monocytogenes into the allantoic sac of 10-day-old embryos led to death within 2–5
days, and the LD 50 was less than 6 × 10
2 cells for virulent strains. L. ivanovii is also lethal by this
method. Injections of 100–30,000 cells/egg into the chorioallantoic membrane of 10-day-old chick
embryos resulted in death within 72 hours compared to about 5 days for mice.
123 Although Anton’s
test and chick embryos may be used to assess the relative virulence of strains of listeriae, the mouse
is the model of choice for the additional information that it gives relative to cellular immunity.
Not only the mouse is the most widely used laboratory animal for virulence studies of listeriae,
but also it is widely used in studies of T cell immunity in general. This model is employed by use of
normal, baby, juvenile, or adult mice, as well as a variety of specially bred strains such as athymic
(T cell-deficient) nude mice. Listerial cells have been administered intraperitoneally (IP), intravenously
(IV), and intragastrically (IG). When normal adult mice are used, all smooth and hemolytic strains
of L. monocytogenes at levels of 10
3 –10
4 /mouse multiply in the spleen.
59 With many strains, inocula
of 10
5 –10
6 are lethal to normal adult mice, although numbers as high as 7 × 10
9 have been found
necessary to produce an LD 50 . A low of 50 cells for 15-g mice has been reported (see below).
Whereas the IP route of injection is often used for mice, IG administration is employed to assess
gastrointestinal behavior of listeriae. The administration of L. monocytogenes to 15-g mice by the
IG route produced more rapid infection and more deaths in the first 3 days of the 6-day test than
by IP.
105 By this method, the approximate 50% lethal dose (ALD 50 ) ranged from 50 to 4.4 × 10
5
cells for 15 food and clinical isolates of L. monocytogenes.
105 Six- to eight-week-old mice were given
IP and oral challenges of a serovar 4b strain by one group to study their effect under normal and
compromised states. Cells were suspended in 11% nonfat milk solids and administered to four groups
of mice: normal, hydrocortisone treated, pregnant, and cimetidine treated. Minimum numbers of cells