
counts on media with and without sodium chloride
addition. Using this technique, the effect of various
agents on the recovery of sublethally injured L. mono-
cytogenes has been determined. Selective agar used
for isolation of Listeria spp. generally did not allow
resuscitation of heat- or acid-injured cells. However,
freezing did not appreciably affect the ability of test
strains to grow on selective agar. The inability of a
medium to support colony formation by thermally
injured L. monocytogenes could be correlated with
the inclusion of phenylethanol, tellurite, polymyxin
B, sodium thiosulfate and oxgall, as well as sodium
chloride. The addition of Tween 80, bovine fetal
serum or egg yolk emulsion to modified Vogel–
Johnson medium increased the ability to detect heat
injured cells of the organism 100-fold without any
loss of selectivity of the medium.
0025 The universal response of cells undergoing injury
appears to be the accumulation of hydrogen peroxide,
and injured cells have an increased sensitivity to
the toxic effects of hydrogen peroxide. Removal of
hydrogen peroxide by the addition of catalase, or the
nonenzymatic peroxide decomposer sodium pyru-
vate, has a beneficial effect on the recovery of injured
cells of several microorganisms, including Staphylo-
coccus aureus and Salmonella typhimurium. Effective
repair of stressed L. monocytogenes cells on solid
medium containing catalase has been described.
Also, pyruvate addition to modified McBride agar
enhanced recovery of heat-injured L. monocytogenes,
but it was necessary to incubate plates at 22–25
C for
7 days in order to achieve beneficial effects. However,
the situation is confused as another study failed to
show enhanced recovery of L. monocytogenes Scott
A strain after addition of pyruvate to modified
McBride agar. The addition of reducing agents to
the medium has been shown to have a mixed effect
on the recovery of heat-injured cells of L. monocyto-
genes, but recovery is increased if cells are incubated
anaerobically. Enrichment in broth containing
oxygen scavengers, such as oxyrase, enabled recovery
of heat-injured listeriae within 6 h.
0026 When the effects of incubation temperature,
oxygen availability and supplementing the culture
medium with magnesium chloride, magnesium sul-
phate, d-glucose, l-cysteine, catalase, or lithium
chloride on the recovery of salt- or acid-damaged
Listeria monocytogenes serovar 1/2c was studied on
a solid repair medium, results showed that conditions
promoting resuscitation of acid- or salt-injured cells
are stress-specific, and differ in part from those
described for heat-stressed Listeria.
0027 It is strongly recommended that a nonselective
enrichment is included when processed foods are
analyzed for L. monocytogenes. An incubation time
of 6–9 h at temperatures in the range of 20–40
Cis
required for complete recovery of injured cells. The
temperature optimum appears to be 25
C. The
organism does not appear to be able to repair at low
temperatures. Prevention of large pH changes during
the enrichment by buffering of the medium also
promotes recovery of heat-stressed cells.
Detection in Suspected Food Poisoning
0028Confirmation of the involvement of L. monocytogenes
in a case of illness suspected to be of food origin relies
on isolation of identical strains of the organism from
the patient and the suspected food. In practice, this is
not feasible in the majority of cases, and epidemi-
ological case-control studies have proved crucial in
implicating a specific food product in an outbreak.
The epidemiological surveillance of listeriosis has
been hampered by the lack of adequate typing
schemes. A serotyping system is available, but is of
limited value as a high proportion of strains causing
disease belong to only a small number of serovars. Of
all L. monocytogenes strains isolated in Britain from
human cases of listeriosis, 91% comprised only three
serovars: 4b (59%), 1/2a (18%), and 1/2b (14%). A
phage-typing scheme is also used, but a high propor-
tion of strains (about 36% in Britain) are not typable
by this system. An international multicenter typing
study of L. monocytogenes was initiated by the
WHO (Food Safety Unit, Geneva) in order to evaluate
the usefulness of various phenotypic and genotypic
typing methods for L. monocytogenes and to select
and standardize the most appropriate methods. Sev-
eral genotyping methods were compared, including
pulsed field gel electrophoresis (PFGE) coupled
with the use of the restriction enzymes, ApaI and
SmaI, random amplification of polymorphic DNA
(RAPD), high frequency restriction endonuclease an-
alysis (REA), and restriction-fragment length poly-
morphism (RFLP). In general, this study reconfirmed
that PFGE is a very accurate and reproducible method
for fine structure comparison and molecular typing of
L. monocytogenes. An automated genotyping system,
the RiboPrinter, has proved useful for the analysis of
L. monocytogenes and was used successfully to deter-
mine the source of the outbreak linked to hot dogs
produced at the Bil Mar plant of Sara Lee. L. mono-
cytogenes has been classified into three genetic lin-
eages using ribotyping and PCR-RFLP typing of
hlyA and octA genes. A higher proportion of human
isolates (69.1%) than industrial isolates (36.8%) were
classified as lineage I, which contains human sporadic
isolates and all epidemic isolates. All other industrial
isolates (63.2%) were classified as lineage II, which
contains only human sporadic isolates.
3578
LISTERIA
/Detection