Foodborne Listeriosis
603
Effect of Sublethal Heating on Thermotolerance
It is unclear whether sublethal heating of L. monocytogenes cells renders them more resistant
to subsequent thermal treatments. Some investigators have reported no effect,
11,13 and others have
reported increased resistance.
35,38,79 In one study, the heat shocking of strain Scott A at 48
◦ C for 20
minutes resulted in a 2.3-fold increase in D values at 55
◦ C.
79 In another study employing Scott A
in broth and ultrahigh temperature (UHT)-treated milk, an increase in heat resistance was observed
following exposure to 48
◦ C for 60 minutes and subsequent exposure to 60
◦ C.
38 Finally, in a study
employing 10 strains at a level of about 10
7 /g in a sausage mix and heat shocking at 48
◦ C for
30 or 60 minutes, no significant increase in thermotolerance was observed at 62
◦ or 64
◦ C, but those
shocked for 120 minutes did show an average 2.4-fold increase in D values at 64
◦ C.
35 In this study, the
thermotolerance was maintained for at least 24 hours when the cells were stored at 4
◦ C. If sublethal
heating does lead to greater thermoresistance, it would not pose a problem for milk that contains fewer
than 10 cells/ml assuming that a twofold to threefold increase in D value occurs.
VIRULENCE PROPERTIES
Of listerial species, L. monocytogenes is the pathogen of concern for humans. Although L. ivanovii
can multiply in the mouse model, it does so to a much less degree than L. monocytogenes, and up to 10
6
cells caused no infection in the mouse.
59 L. innocua, L. welshimeri, and L. seeligeri are nonpathogens,
although the last produces a hemolysin. The most significant virulence factor associated with
L. monocytogenes is listeriolysin O (LLO).
Listeriolysin O and Ivanolysin O
In general, the pathogenic/virulent strains of L. monocytogenes produce beta-hemolysis on blood
agar and acid from rhamnose but not from xylose. Strains whose hemolysis can be enhanced with
either the prepurified exo-substance or by direct use of the culture are potentially pathogenic.
117
Regarding hemolysis, the evidence is overwhelming that all virulent strains of this species produce
a specific substance that is responsible for beta-hemolysis on erythrocytes and the destruction of
phagocytic cells that engulf them. The substance and perfringolysin O (PFO) have been shown to
be highly homologous to streptolysin O (SLO) and pneumolysin O (PLO). It has been purified and
shown to have a molecular weight of 60,000 Da and to consist of 504 amino acids.
44,95 It is produced
mainly during the exponential growth phase, with maximum levels after 8–10 hours of growth.
43 Less
LLO is synthesized at 26
◦ C than at 37
◦ C with high glucose, and synthesis was found to be best with
0.2% glucose at 37
◦ C.
24 Sorbate at a level of 2% inhibited LLO synthesis at 35
◦ C under aerobic or
anaerobic conditions.
72 LLO has been detected in all strains of L. monocytogenes, including some
that were nonhemolytic, but not in L. welshimeri or L. grayi. The gene that encodes its production is
chromosomal and it is designated hly. Its role in virulence is discussed below.
L. ivanovii and L. seeligeri produce thiol-dependent exotoxins that are similar but not identical to
LLO. Large quantities are produced by L. ivanovii but only small quantities by L. seeligeri.
43 The L.
ivanovii thiol-activated cylolysin is ivanolysin O (ILO). Antiserum raised to the L. ivanovii product
cross-reacts with that from L. monocytogenes and SLO.
73 ILO-deficient mutants have been shown to
be avirulent in mice and chick embryos.
1
603
Effect of Sublethal Heating on Thermotolerance
It is unclear whether sublethal heating of L. monocytogenes cells renders them more resistant
to subsequent thermal treatments. Some investigators have reported no effect,
11,13 and others have
reported increased resistance.
35,38,79 In one study, the heat shocking of strain Scott A at 48
◦ C for 20
minutes resulted in a 2.3-fold increase in D values at 55
◦ C.
79 In another study employing Scott A
in broth and ultrahigh temperature (UHT)-treated milk, an increase in heat resistance was observed
following exposure to 48
◦ C for 60 minutes and subsequent exposure to 60
◦ C.
38 Finally, in a study
employing 10 strains at a level of about 10
7 /g in a sausage mix and heat shocking at 48
◦ C for
30 or 60 minutes, no significant increase in thermotolerance was observed at 62
◦ or 64
◦ C, but those
shocked for 120 minutes did show an average 2.4-fold increase in D values at 64
◦ C.
35 In this study, the
thermotolerance was maintained for at least 24 hours when the cells were stored at 4
◦ C. If sublethal
heating does lead to greater thermoresistance, it would not pose a problem for milk that contains fewer
than 10 cells/ml assuming that a twofold to threefold increase in D value occurs.
VIRULENCE PROPERTIES
Of listerial species, L. monocytogenes is the pathogen of concern for humans. Although L. ivanovii
can multiply in the mouse model, it does so to a much less degree than L. monocytogenes, and up to 10
6
cells caused no infection in the mouse.
59 L. innocua, L. welshimeri, and L. seeligeri are nonpathogens,
although the last produces a hemolysin. The most significant virulence factor associated with
L. monocytogenes is listeriolysin O (LLO).
Listeriolysin O and Ivanolysin O
In general, the pathogenic/virulent strains of L. monocytogenes produce beta-hemolysis on blood
agar and acid from rhamnose but not from xylose. Strains whose hemolysis can be enhanced with
either the prepurified exo-substance or by direct use of the culture are potentially pathogenic.
117
Regarding hemolysis, the evidence is overwhelming that all virulent strains of this species produce
a specific substance that is responsible for beta-hemolysis on erythrocytes and the destruction of
phagocytic cells that engulf them. The substance and perfringolysin O (PFO) have been shown to
be highly homologous to streptolysin O (SLO) and pneumolysin O (PLO). It has been purified and
shown to have a molecular weight of 60,000 Da and to consist of 504 amino acids.
44,95 It is produced
mainly during the exponential growth phase, with maximum levels after 8–10 hours of growth.
43 Less
LLO is synthesized at 26
◦ C than at 37
◦ C with high glucose, and synthesis was found to be best with
0.2% glucose at 37
◦ C.
24 Sorbate at a level of 2% inhibited LLO synthesis at 35
◦ C under aerobic or
anaerobic conditions.
72 LLO has been detected in all strains of L. monocytogenes, including some
that were nonhemolytic, but not in L. welshimeri or L. grayi. The gene that encodes its production is
chromosomal and it is designated hly. Its role in virulence is discussed below.
L. ivanovii and L. seeligeri produce thiol-dependent exotoxins that are similar but not identical to
LLO. Large quantities are produced by L. ivanovii but only small quantities by L. seeligeri.
43 The L.
ivanovii thiol-activated cylolysin is ivanolysin O (ILO). Antiserum raised to the L. ivanovii product
cross-reacts with that from L. monocytogenes and SLO.
73 ILO-deficient mutants have been shown to
be avirulent in mice and chick embryos.
1