Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set
Подождите немного. Документ загружается.
may be a good route for recycling these bacteria.
L. monocytogenes was isolated from 62% of fresh
or low-salinity waters in tributaries draining into an
estuary impacted by humans and domesticated and
wild animals. One L. monocytogenes serogroup
appeared to predominate in fresh water when domes-
ticated animals were nearby, but a greater variety of
strains with no predominance by a single serogroup
was observed in areas with no direct animal influ-
ence. The majority of strains found on decaying
plants are not animal pathogens and this suggests
that the predominance of certain serotypes of
L. monocytogenes may depend on environmental
factors.
Presence in Food
0009 L. monocytogenes has been isolated from a wide
variety of foods of animal and plant origin (Table 1).
The incidence of the bacterium in foods will be de-
scribed on a commodity basis.
Occurrence in Milk and Dairy Products
0010 Because of the well-documented foodborne out-
breaks of listeriosis associated with the consumption
of dairy products, these foodstuffs have attracted the
attention of many researchers. Several studies have
been carried out across the world to assess the inci-
dence of L. monocytogenes in raw milk. Five large
studies were carried out in Pennsylvania, USA (2511
samples), England and Wales (2009 samples), On-
tario, Canada (1720 samples), France (1409 samples),
and Transvaal, South Africa (961 samples). These
studies revealed the incidence of L. monocytogenes
to be 3.1, 5.1, 2.7, 6.0, and 5.2%, respectively. Col-
lectively these investigations found that 363 of 8610
samples or 4.2% of raw milks were positive for
L. monocytogenes. The level of the organism in raw
milk is usually less than 1 cfu ml
1
but counts may
reach 64 cfu ml
1
. The source of L. monocytogenes in
the milk supply is still in some doubt. L. mono-
cytogenes is a causative agent of mastitis, albeit only
rarely. The organism has been recovered from milk of
mastitic cows at counts ranging from 10
3
to 10
4
cfu ml
1
. Prolonged excretion of L. monocytogenes
in milk and feces of infected animals and asymptom-
atic carriers has been described. This would lead to
the build-up of an endogenous pool of Listeria in the
environment which could be augmented from other
sources such as the use of poor-quality silage and
feces of feral animals and birds. In a large case-
control study involving 128 dairy farms in France,
poor-quality silage (pH > 4), inadequate frequency of
cleaning the exercise area, poor cow cleanliness, in-
sufficient lighting of milking barns and parlors, and
incorrect disinfection of towels between milkings
were associated with contamination of milk by
L. monocytogenes.
0011Numerous other dairy products have been found to
contain L. monocytogenes. The bacterium has been
found in pasteurized milk and an outbreak of listeri-
osis in Massachusetts in 1983 was associated with the
consumption of this product. One report from Spain
suggested that 21% of pasteurized milk samples tested
tbl0001Table 1 Occurrence of Listeria monocytogenes in some foods
Food type Samples contaminated with
L. monocytogenes (%)
Dairy
Raw milk 0–45.3
Pasteurized milk 0–21.4
Flavored milks 0.8–3.3
Creams 0
Soft and semisoft cheeses 0–61
Hard cheeses 0–9
Icecream 0.3–6.1
Butter 0
Yogurt 0
Powder 0
Sheep and goats’ milk 0.1–2.9
Meat and poultry
Raw beef 0–30.9
Raw lamb 0–60
Raw pork 0–68
Ground meats 11.7–92
Ham 11.8–33.8
Sausages 15.7–88
Fermented, dried sausages 0–56.7
Frankfurters 7.5
Processed meats 0–72.2
Pa
ˆ
te
´
2.76–10.7
Raw poultry 3–58.6
Cooked poultry 0–27.2
Eggs 0
Seafood
Raw fish 0–33.3
Smoked salmon 0–44.4
Uncooked shellfish 12.1–25.4
Cooked shellfish 0–28.1
Vegetables
Beansprouts 12.1–85.7
Cabbage 1.1–2.1
Cucumber 2.2–80
Potatoes 21.2–27.1
Prepacked salads 0–14.3
Radishes 14.4–36.8
Salad vegetables 2.3–44
Spring rolls 62.5
Tomatoes 13.3
Vegetables 6.2–12.2
Frozen vegetables 2.2
Other
Ready-prepared meals 2.9–13.2
Frozen meals 11.9
Vegetarian meals 31.8
Mayonnaise 4.2–5.2
LISTERIA
/Properties and Occurrence 3563
were positive for the organism. However, these milks
showed evidence of gross contamination with other
bacteria, indicative of poor hygiene conditions in
the processing plant and of postpasteurization con-
tamination, e.g., at bottling/packing. More reliable
estimates put the level of contamination of pasteur-
ized milk with L. monocytogenes at < 1%. Other
fluid milk products, such as chocolate milk, can
have L. monocytogenes present. The isolation of
L. monocytogenes from pasteurized milk prompted
investigators to suggest that, because of its intracellu-
lar location, it was able to survive pasteurization.
However, many studies have now confirmed that
this is not the case (Figure 1).
0012 Dairy products most at risk of contamination with
L. monocytogenes are soft cheeses such as Camembert
and Brie. Again, numerous surveys have been carried
out worldwide to determine the incidence of listerial
contamination of these products. These studies
have revealed a widespread distribution of the
organism with L. monocytogenes counts in the range
1 10
2
to 1 10
6
g
1
. In a study carried out in
the Netherlands, all positive cheeses were manufac-
tured from raw milk, but other work has shown no
difference in the incidence of L. monocytogenes in
cheese produced from raw or pasteurized milk. The
contamination of Swiss cheese with L. monocytogenes
was primarily due to the presence of the organism on
shelves used for maturation of the cheeses. On rare
occasions, hard cheeses, including Cheddar, have
become contaminated with the bacterium.
0013 Among other dairy products shown to harbor
L. monocytogenes are icecream and butter, although
there have been no documented cases of listeriosis
where icecream was the vehicle of infection.
0014 Since the recognition of the link between listeriosis
and consumption of contaminated milk products,
research has focused on the source of this contamin-
ation. In one such study the incidence of Listeria
contamination of processing equipment was com-
pared with that of the general dairy-processing envir-
onment. Positive equipment samples were obtained
from six of the 21 (28.6%) plants, whereas 19 of the
21 (90.5%) plants had positive environmental sites.
L. monocytogenes was isolated from 11 of the 215
(5.1%) samples taken from equipment, and these
included three holding tanks, two table tops, three
conveyor-chain systems, a pasta filata wheel, a pint
milk filler and a brine prefilter machine. Twenty-four
of the 163 (14.7%) samples from environmental sites
were positive for L. monocytogenes. These results
suggest that environmental contamination with
Listeria does not necessarily translate into contamin-
ation of equipment within the same dairy plant.
Sites that are typically positive for L. monocytogenes
include drains, conveyor belts, floor mats and foot
baths, coolers, freezers, areas underneath equipment,
areas associated with case washers, areas where raw
milk is handled, cheese-ripening rooms, and cheese-
washing brushes.
0015In addition, the processing environments of 30
dairies were surveyed for the presence of Listeria
spp. Nine of these dairies had a dairy farm in close
proximity to the processing facilities, and these plants
were more likely to be contaminated (9/9) than the
plants without on-site dairy farms (17/21). Plants
producing dairy ingredients, frozen dairy products,
or fluid milk had significantly higher incidence rates
than expected, whereas plants producing fermented
dairy products, or a combination of fermented dairy
products and fluid milk, had significantly lower inci-
dence rates than expected. There was no significant
difference in contamination rates between areas
within the plants.
Occurrence in Meat and Meat Products
0016L. monocytogenes has been isolated from beef, lamb,
pork, turkey, and chicken with high frequency
(Table 1). There is considerable evidence that the
contamination of meat arises from environmental
sources which can occur at all points along the
processing and retail chains. Common sources of
contamination are the environment of the chilling
and cutting rooms, hands, and equipment. Cross-
contamination may also occur between raw and
cooked product, and from contact of cooked prod-
ucts with soiled surfaces due to inadequate cleaning
and disinfection.
0017The source of contamination can vary depending
on meat species. For beef and lamb, the hide seems to
be a more important source of carcass contamination
by the bacterium than feces, but for pork, fecal con-
tamination is more important. Poultry carcasses can
become contaminated from the environment of pro-
cessing plants or due to carriage of the organism in
the gut of healthy birds. L. monocytogenes can con-
taminate poultry at several points during production,
including at evisceration due to spillage of gut con-
tents, from handling, from equipment such as the
rubber fingers used for plucking, and from the scald
tank water. L. monocytogenes can persist in the en-
vironment of poultry-processing plants for up to
1 year.
0018Levels of L. monocytogenes found in cooked
poultry products can be high (> 10
4
cfu g
1
) and this
may be the result of inadequate cooking or growth of
the bacterium arising from contamination after the
cooking step. However, most studies have shown that
when Listeria is present in meats it is usually found in
low numbers. The level of contamination is generally
3564
LISTERIA
/Properties and Occurrence
higher for poultry than pork, which, in turn, is higher
than in beef. There is evidence that L. monocytogenes
apparent numbers are affected by the microbiological
methods applied, especially where cells might be in-
jured.
0019 L. monocytogenes is widely distributed in meat-
processing plants and the conditions present in these
locations are ideal to promote the growth of the
organism. This makes it difficult, but not impossible,
to control contamination of the meat.
Occurrence in Fish and Seafood
0020 Because of the many routes by which Listeria can
contaminate water, it is not surprising that L. mono-
cytogenes has been isolated from fish and seafood at
high prevalence rates. The organism can become
widely disseminated throughout smokehouses during
production of smoked salmon and L. monocytogenes
can be isolated from many sites in a raw-shrimp-
processing plant. These include processing water,
equipment, food handlers, doors, walls, floors, and
drains. Thus, it is essential to institute a regular
cleaning and disinfection program within these
operations.
0021 A study in the UK which examined 4435 seafood
samples found that L. monocytogenes could be isol-
ated from 128 (3%) of these samples. The organism
was most frequently isolated from smoked fish and
fish pa
ˆ
te
´
. The extent of contamination was generally
low, but in eight samples L. monocytogenes counts
exceeded 10
2
cfu g
1
.
Occurrence in vegetables
0022 As has been previously stated, L. monocytogenes
is found in many habitats, including soil, water,
manure, decaying vegetation, and sewage effluents.
This provides many opportunities for the contamin-
ation of vegetables by this bacterium. Despite this
potential for contamination, surprisingly few types
of vegetables are consistently found to contain
L. monocytogenes. These are mainly potatoes and
radishes. The incidence of the organism on fresh-cut
vegetables varies between 0 and 19%, and when
L. monocytogenes is detected it is present in low
numbers (< 2 10
2
cfu g
1
).
Occurrence in Processing Plants and Kitchens
0023 Listeria spp. are widely distributed in the environ-
ment of food-processing plants (Table 2). Common
sources of these bacteria include drains, floors, stand-
ing water, and food residues. The absence of Listeria
spp. from dry culinary food units prompted one
group of researchers to conclude that dry conditions
and the absence of food residues contribute to the
control of L. monocytogenes. Codes of practice for
the food industry have been set out to help eliminate
problems caused by L. monocytogenes. These entail a
rigid program encompassing personal hygiene, separ-
ation of raw material and product, and effective
clean-up and disinfection procedures. These, coupled
with the adoption of a hazard analysis critical control
point (HACCP) management system operated by
trained and knowledgeable staff, should help to
insure the microbiological safety of foods.
0024Even when the product entering the home is free
from contamination, there is considerable scope for
mishandling foods in domestic kitchens. One survey
has reported that 20% of domestic environments are
contaminated with Listeria spp., with dishcloths
being the main culprits. L. monocytogenes was also
isolated from a domestic refrigerator.
Fate During Processing
Temperature
0025The identification of pasteurized milk as a vehicle in
the transmission of listeriosis led to the assumption
that L. monocytogenes was unusually heat-resistant.
It was thought that this resistance was mediated by its
intracellular location within the leukocytes found
in milk. This provoked much interest in the ther-
mostability of the organism. It is now commonly
accepted by most researchers that it is unlikely that
L. monocytogenes can survive commercial pasteur-
ization conditions in appreciable numbers and there
is little difference in the heat resistance of free and
intracellularly located cells.
tbl0002Table 2 Occurrence of Listeria spp. and L. monocytogenes in
environmental samples taken from food-processing plants
a
Samplingpoint Samples positive (%) for
Listeria spp. L. monocytogenes
Drains 21 7
Floors 43 4
Condensate/stagnant
water
11 1
Residues 25 5
Processing equipment/
food contact surfaces
30 19
Conveyors 53 26
Effluent 69 47
Miscellaneous
b
16 3
All samples 18 6
a
Environmental samples taken from milk-processing factories (including
fluid milk, cheese, icecream, cultured products, and powder plants),
frozen food, meat (including poultry)-processing and potato-processing
sites.
b
Miscellaneous sampling sites included wooden pallets, soil, and cleaning
materials.
LISTERIA
/Properties and Occurrence 3565
0026 Results of several researchers who examined the
effect of heat on L. monocytogenes have been ana-
lyzed and, in general, the heat stability of L. mono-
cytogenes can be described by the equation:
log D ¼ 13:64 0:19 T ð1Þ
where D is the time in seconds to achieve a 10-fold
reduction in bacterial number, and T is the heating
temperature in degrees Celsius. Representative
D-values at a variety of temperatures and in different
foods are given in Table 3.
0027 The heat resistance of this bacterium is increased
after sublethal heat shock. This increase in resistance
due to sublethal injury can also be triggered by prior
exposure to other stressors, such as low pH. As well,
the heat resistance of L. monocytogenes may be in-
creased severalfold on cured meats, and this increase
may not be entirely due to changes in water activity
(a
w
) in the presence of curing salts.
0028 The method used to recover heat-stressed cells can
also have an impact on the apparent heat-resistance
of the bacterium. For example, the use of anaerobic
conditions leads to greater recovery of heat-stressed
cells than culture in the presence of oxygen.
0029The organism is resistant to freezing. The viable
populations of four strains of L. monocytogenes
decreased by only 3–6% after 14 days’ storage at
18
C. In another study, the effects of freezing and
subsequent frozen storage at 18
C on the viability
of a mixed inoculum of seven L. monocytogenes
strains in ground beef, ground turkey, frankfurters,
canned corn, icecream mix, and tomato soup was
determined. Listeria responses were related to the
pH of the food. Where the pH of the food was
> 5.8, L. monocytogenes survived freezing and stor-
age well. However, in tomato soup, where the pH was
4.74, the organism showed a decline in viability
during prolonged storage and could not be quantita-
tively recovered on selective media. L. monocytogenes
can also survive in seafood during storage at 20
C
for 3 months. Under these conditions populations on
the frozen products decreased by less than 1 log after
3 months.
Sanitizers
0030A number of approved sanitizers, including acid an-
ionic detergents, quaternary ammonium compounds,
carrier-bound iodine, and chlorine, all reduced
L. monocytogenes populations by a minimum of
99.999% at the recommended doses. Products show-
ing a high efficacy contained iodine, peroxide, or
quaternary ammonium as the active agent. However,
the efficiency of sanitizers can be reduced in the pres-
ence of organic material. There is also evidence that
repeated exposure to quaternary ammonium com-
pounds can result in the acquisition of resistance to
the sanitizer. The mechanism of action appears to be
by the use of efflux pumps which can reduce intracel-
lular concentrations of the sanitizer.
0031In addition, Listeria spp. can attach to processing
surfaces where they may form biofilms. Cells present
in biofilms have increased resistance to desiccation
and the effects of disinfectants. Resistance of adher-
ent cells to sanitizers is dependent upon the surface to
which they are attached. Complete biofilm removal
and/or inactivation was obtained in many cases
where the surface was first cleaned prior to exposure
to sanitizer.
Survival in Foods
0032L. monocytogenes has the ability to survive in a wide
variety of foodstuffs (Table 4). The main factors
affecting survival appear to be pH, salt concentration,
and a
w
, although the resistance of the organism to
drying is also affected by temperature and relative
humidity, as well as the nature of the suspending
menstruum. Survival times are generally longer at
lower temperatures. The organism can survive pH
tbl0003 Table 3 Heat resistance of Listeria monocytogenes in various
products
Product Temperature
(
C)
D-value
(min)
z-value
(
C)
Ground meat 60 3.12 5.3
Fermented sausage 60 9.2–16.7 4.6
Roast beef 60 3.5–4.5 7.2
Beef 60 3.8
Beef 70 0.14
Beef homogenate 60 6.3–8.3
Naturally
contaminated beef
60 1.6
Beef steak 60 6.3–8.3 6.0
Beef steak 70 0.1–0.2
Weiner batter 60 2.3
Chicken leg 60 5.6 6.7
Chicken leg 70 0.11
Chicken breast 60 8.7 6.3
Chicken breast 70 0.13
Chicken 60 5–5.3 6.7–7.4
Chicken 70 0.16–0.2
Chicken homogenate 60 5.0–5.3
Raw liquid whole egg 60 1.5 6.6
Crabmeat 60 2.6 8.4
Mussels 60 5.5 4.3
Crawfish tail meat 60 2.0 5.5
Salmon fillets 60 4.2–4.5 5.6–6.7
Cod fillets 60 2.0 5.7–6.1
Lobster 60 2.4 5.0
Carrot homogenate 60 5.0–7.8
Milk and cream 52.2 24.1–52.8 5.5
Milk and cream 57.8 4.0–8.2
Milk and cream 63.3 0.2–0.6
Milk and cream 66.1 0.1–0.3
3566
LISTERIA
/Properties and Occurrence
tbl0004 Table 4 Survival of Listeria monocytogenes in environmental and food samples
Product Approximate
inoculum size
(cfu ml
1
or g
1
)
Storage
temperature
(
C)
Final pH Change in
cell numbers
a
Survival
period
(days)
Environmental samples
Cooling water 10
4
4 þ/– > 70
Tiled surface 10
8
??>30
Waxed cardboard/ plastic cartons – 0.8/6.6 – 2–14
Dairy products
Raw milk
b
10
4
4–þþþ >16
Fermented milk 10
3
–10
4
4 4.2–4.7 – – – 14–49
Buttermilk 10
4
4 4.4 – – 18–26
Evaporated milk 10
4
–10
7
7/21 – þþþ 28–56
Sweetened condensed milk 10
4
–10
7
7/21 – – > 42
Yogurt 10
2
–10
7
4/5 4.1 – – – 1–27
Nonfat dried milk 10
5
–10
6
25 – – – – 28–70
Butter 10
4
–10
5
18 – – > 70
4/6 – þ >70
13 – þþ >70
Icecream 10
0
12 – þ/– >42
Cheeses
Blue 10
5
10 6–6.5 þ >168
Brie 10
5
4 5.5–6 – – >120
Camembert 10
5
67þþþ >56
Cheddar 10
5
6/13 5.1 – – 154–434
Colby 10
5
4 5.1 – >140
Cold pack 10
5
4 5–5.5 þ/– >182
Cottage 10
5
3 5.2–5.3 – – – 3–28
Feta 10
4
4 4.3–4.6 – >90
Goats’ milk 10
6
12 5.5–6.5 þ/– >126
Gouda 10
3
13 6 þ/– >42
Maasdam 10
3
45.5þ/– >42
Manchego type 10
3
–10
5
15 5.1–5.8 þ/– >60
Red smear 10
2
57.4þþþ >91
White brined 10
2
10/12 4.5–5.1 þ/– >42
White mold ripened soft 10
2
57.7þþþ >91
Meat and meat products
Beef
High-moisture 10
4
5–þþþ >42
Low-moisture 10
4
5 – – – >42
Vacuum-packed 10
3
–10
5
0/10 – þþ >50
2–þ/– >60
Carbon dioxide-packed 10
3
–10
5
5/10 – þþþ >21
0/2 – – – – NS
c
Spray-chilled 10
6
5 – – >16
Ground 10
5
–10
7
4 5.6–5.9 þ/– >30
Roast 10
2
–10
2
4.4 4.6–5.3 þ/– >70
Liver 10
5
4–þ/– >30
Chicken, sliced 10
0
–10
3
4.4 6 þþþ >28
Ham 10
1
–10
4
4.4 5.1–6.2 þþþ >42
Lamb ? 10/23 – – – – >20
Turkey, sliced 10
2
–10
3
4.4 5–5.8 þþþ >42
Sausage
Bologna 10
2
–10
2
4.4 5.1–6.2 þþ >42
Bratwurst 10
2
–10
3
4.4 5.4 þþþ >42
Finnish fermented 10
6
10 4.6 – – – >21
Norwegian fermented, dry 10
5
–10
6
4 4.9 – >170
20 4.9 – – – < 170
Pepperoni 10
5
4 4.5 – – – >56
Salami 10
3
–10
4
4 4.3–4.5 – – – >84
Summer 10
2
–10
2
4.4 4.6–5 þ/– >84
Wieners 10
2
–10
3
4.4 4.4–5.8 þþþ >63
Ravioli 10
5
5 – – >14
Egg, whole pasteurized 10
3
4/10 – þþþ >42
Continued
LISTERIA
/Properties and Occurrence 3567
values of 3.6 for up to 25 days if kept at refrigeration
temperatures. However, survival is dependent on the
acidulant used. There are also reports indicating that
some strains of L monocytogenes can survive salt
concentrations as high as 25.5% for at least 32 days
when stored at 4
C.
0033 Because it is essential to minimize recontamination
of processed foods from the environment, processors
must be aware that once L. monocytogenes is present
in a food-processing plant it can persist for long
periods of time (> 136 days), especially if the tempera-
ture is low and the bacterial cells are protected by
residual food components.
Irradiation
0034 In studies carried out using chicken breast meat,
L. monocytogenes was found to be the most resistant
to irradiation when compared to six other common
foodborne pathogens. However, no significant differ-
ences were observed in the radiation resistance of
L. monocytogenes in different meats. The D
10
value
at 5
C was approximately 0.5 kGy for all the meats
tested. Results obtained for L. monocytogenes were
the same regardless of whether irradiation was de-
livered by electron beam or by gamma-rays from a
60
Co source. It is generally accepted that the irradi-
ation dose currently approved by the Food and Drug
Administration for use in meats is sufficient to control
L. monocytogenes.
Other Nonthermal Processing Techniques
0035 In an effort to provide consumers with minimally
processed foods that have retained their nutritional
status, a variety of nonthermal processing techniques
are in various stages of development. These include
high-pressure, pulsed electric fields, oscillating
magnetic waves, high-intensity light, and others.
The effects of a number of these on L. monocytogenes
have been investigated. The ability of the organism to
survive high-pressure processing of a variety of foods,
including milk, cheese, beef, pork, and icecream mix,
has been evaluated. In one study, early-stationary-
phase cells in 1% peptone solution were pressurized
at 345 MPa for 5 min at 25
C or for 5, 10, or 15 min
at 50
C. Viability losses (in decimal reductions)
following pressurization at 25
C ranged from 0.9 to
3.5 among nine L. monocytogenes strains. When two
strains showing the greatest difference in pressure
resistance were treated at 345 MPa and 50
C, both
strains were reduced by greater than eight decimal
reductions within 5 min.
0036In raw milk, pressures greater than 350 MPa did
not permit enumeration of survivors of L. mono-
cytogenes Scott A, due to the rapid fall in numbers.
Although L. monocytogenes had a higher instantan-
eous pressure kill value, it was more resistant during
holding under pressure than natural microflora.
0037Effects of high hydrostatic pressure on two Listeria
monocytogenes strains (NCTC 11994 and a poultry
strain) were examined in different foods at approxi-
mately 18
C. Samples of ultra heat-treated (UHT)
milk, raw and cooked chicken mince, and raw and
cooked beef mince were inoculated with L. mono-
cytogenes and exposed to a pressure of 375 MPa.
Generally, for both strains, increasing the time of
exposure led to a decrease in the number of survivors;
however, resistance did vary depending on the nature of
the food substrate. Survival was significantly greater (P
< 0.01) in UHT milk compared to that in meat, and
survival of both strains was higher in cooked meats
compared to raw meats. Complete inactivation of
strain NCTC 11994 occurred in milk treated with a
pressure of 375 MPa for 15 min at 45
C.
Product Approximate
inoculum size
(cfu ml
1
or g
1
)
Storage
temperature
(
C)
Final pH Change in
cell numbers
a
Survival
period
(days)
Seafood
Shrimp, raw 10
5
20 – ? >90
Fruit and vegetables
Apple cider, unpasteurized 10
5
–10
6
4 3.3–3.5 – – – >21
10 – – – < 7
Asparagus 10
4
4–þ/– >14
Broccoli 10
4
4–þ/– >14
Cabbage 10
4
5–þþþ >64
Cauliflower 10
3
4–þ/– >14
Lettuce 10
4
–10
5
5/25 – þ >14
a
Symbols represent change in L. monocytogenes counts during storage: >3 log
10
increase (þþþ) or decrease (– – –) in count; 1–3 log
10
increase (þþ)or
decrease (– –) in count; 0–1 log
10
increase (þ) or decrease (–) in count; no change in count (þ/–).
b
Naturally contaminated raw milk.
c
NS; no survival.
Table 4 Continued
3568
LISTERIA
/Properties and Occurrence
0038 In order to investigate the effect of pulsed electric
field (PEF), pasteurized whole, 2% and skim milk
were inoculated with L. monocytogenes Scott A
and the effects of milk composition (fat content) and
PEF parameters (electric field strength, treatment
time, and treatment temperature) on the inactivat-
ion of the bacterium were studied. No significant
differences were observed in the inactivation of
L. monocytogenes Scott A in the three types of milk
by PEF treatment. However, the lethal effect of PEF
was a function of field strength and treatment time,
with higher field strength or longer treatment
time resulting in a greater reduction of viable cells.
A 4-log reduction of the bacterium was obtained by
increasing the treatment temperature to 50
C. The
use of PEF is a promising technology for inactivation
of L. monocytogenes, especially when combined with
the addition of nisin.
Factors Affecting Growth
0039 Many factors affect the growth of L. monocytogenes
and, because they interact, they should not be con-
sidered in isolation. Many mathematical models have
been developed that predict the response of the or-
ganism when grown under a variety of conditions.
Some of the important variables governing the
growth of this bacterium are discussed below and
limiting parameters for growth are shown in Table 5.
Temperature
0040 One of the major reasons why L. monocytogenes has
posed such a threat to the food industry is its ability
to grow at refrigeration temperatures. Some strains
of the organism are capable of growth at 0.1 to
0.4
C in UHT milk and chicken broth, but a
study of 100 strains of L. monocytogenes found a
mean minimum growth temperature of 1.1
C. At
refrigeration temperatures (4–5
C) the generation
time for L. monocytogenes varies between about 20
and 60 h, depending on the food in which it is present.
At abusive temperatures (10
C) the organism will
grow with doubling times of 10–15 h.
0041 The previous growth temperature of the inoculum
can have a marked effect on both rate and extent of
growth at low temperatures. Prior acclimatization of
the bacterium at low temperatures reduces the lag
time for growth.
0042There is evidence that virulence of L. monocyto-
genes is enhanced by growth at low temperatures.
pH
0043Listeria spp. grow at a wide range of pH (5–9.5)
which encompasses the pH of most foods. The min-
imum pH at which growth occurs is inversely propor-
tional to the growth temperature and growth at pH
values below 5 has been observed at temperatures
near the optimum for growth (30
C). The nature of
the acidulant also affects growth. In terms of concen-
tration of undissociated acid required to inhibit
growth, lactic and citric acids are the most effective
inhibitors of the organism.
0044Other factors, such as a
w
, alter the response of
Listeria spp. to pH changes. The inhibitory effects
of salt and pH appear to be additive.
Water Activity and Osmotic Pressure
0045Growth of L. monocytogenes is inhibited in media
with an a
w
of less than 0.942 (achieved by the add-
ition of sucrose or sodium chloride) or 0.932 (by the
addition of glycol). In one study involving the add-
ition of sucrose to broth, growth of L. monocytogenes
was observed at an a
w
of 0.92. Proliferation of the
organism was observed in 12% sodium chloride at
pH 7 and 30
C, but the concentration of sodium
chloride supporting growth increased with a decrease
in temperature.
0046L. monocytogenes can grow in high-osmotic-
strength environments. A number of compounds,
including glycine betaine, and carnitine, act as osmo-
protectants, and their addition to defined medium
causing osmotic stress resulted in an increase in
growth rate of L. monocytogenes 10403S and Scott
A. These same compounds also act as cryoprotect-
ants, as, when added to media, they increased the
growth rate of L. monocytogenes at 5
C.
Substrate
0047The rate of growth of L. monocytogenes is remark-
ably similar in a variety of foodstuffs (Table 6). This
may be expected, considering the pH and a
w
values
for these foods are similar. When slices of cooked
beef, pork, chicken, or turkey were inoculated with
L. monocytogenes strain Murray B, vacuum-
packaged, and stored at 5
C, decreases in pH (6.9
to 5.9) and a
w
(0.993 to 0.960; adjusted with NaCl)
of the cooked meats increased the lag time and re-
duced the growth rate of the bacterium, but the type
of meat had no effect on the growth of the organism
after allowance was made for pH. Thus, slower
tbl0005 Table 5 Limiting growth conditions for Listeria monocytogenes
a
Factor Minimum Optimum Maximum
Temperature (
C) 0.4–0.1 37 44
pH 4.3–5.7 7 8
Water activity 0.9–0.96 – –
a
Values vary depending on strain, medium, humectant, acidulant, and
interactive effects between growth conditions.
LISTERIA
/Properties and Occurrence 3569
growth could be expected in fermented foods, al-
though other factors may also contribute to the rate
of Listeria replication in foods. For example, when
growth of L. monocytogenes on raw and cooked beef,
chicken meat, catfish, and shrimp at 4
C was com-
pared, L. monocytogenes grew faster and reached a
higher population on raw and cooked catfish and
shrimp than on beef or chicken. Some difference in
growth rate was due to inherent pH differences of
fresh tissues, but on cooked tissues, growth rate on
acidified shrimp was the same as that on shrimp with
the normal pH of 7.6. Growth rates of Listeria on
cooked beef at either pH were similar to those noted
on uncooked tissue. This indicates that additional
factors in shrimp exert inhibitory effects at reduced
pH levels, and that these may be heat-related.
0048 The fat content of milk may dictate growth rates
and population levels for L. monocytogenes strains of
serotype 4b. However, other work failed to confirm
this enhanced growth with increasing fat levels. Better
growth of L. monocytogenes on fatty meat rather
than lean tissue has also been found, and this was
reportedly due to a shorter lag phase of growth on the
fat.
0049 L. monocytogenes was able to grow to greater cell
numbers in chocolate milk than in skim or whole
milk. The growth of the organism in milk was also
enhanced by the addition of sugar, cocoa, and carra-
geenan stabilizer. The major substrate supporting the
growth of L. monocytogenes in milk appears to be
glucose.
0050 The characteristics of L. monocytogenes isolated
from different foods appear to be different.
Modified Atmospheres
0051In a study on cooked chicken loaf stored under a
modified atmosphere containing 50% carbon dioxide
and 10% oxygen, growth of L. monocytogenes was
retarded at 3 and 7
C but not at 11
C. Modification
to obtain an atmosphere of 80% carbon dioxide and
0% oxygen resulted in decreased growth at all three
storage temperatures. Similar results have been
reported for beef, pork, lamb, and turkey packed
under carbon dioxide when growth was observed at
10
C, but was inhibited at temperatures between 0
and 5
C. In general, for L. monocytogenes present in
modified-atmosphere-packaged raw meats, growth is
decreased with increasing carbon dioxide concentra-
tions and decreasing temperature. Growth of
L. monocytogenes occurs in beef and cooked chicken
packaged under vacuum and in oxygen-permeable
or -impermeable films.
0052To investigate the effect of modified-atmosphere
packaging on processed meats, samples of sliced
frankfurters were inoculated with L. monocytogenes
and packaged under atmospheres containing between
20 and 80% carbon dioxide (the remainder being
nitrogen). Only the 80% carbon dioxide atmosphere
fully inhibited L. monocytogenes at 4, 7, and 10
C,
but this carbon dioxide concentration caused un-
acceptable souring of the sample.
0053Modified-atmosphere packaging also has little
effect on the ability of L. monocytogenes to grow in
fish and seafood or in fresh-cut vegetables such as
lettuce. However, there is evidence to suggest that
bacteriocin-forming lactic acid bacteria or bacterio-
cin isolates may be useful as biopreservatives for
modified-atmosphere-packaged vegetables.
Inhibitors
0054A number of naturally occurring compounds and
food additives are inhibitory to L. monocytogenes.
These include the enzymes lactoperoxidase (found in
milk) and lysozyme (milk and egg white). Lactoper-
oxidase enhances thermal destruction of the bacter-
ium. Milk lactoferrins also inhibit the organism and it
is speculated that an animal tissue bacteriostatic
agent active against L. monocytogenes exists.
0055Fatty acids, naturally occurring in milk from sev-
eral mammalian species, are inhibitory for L. mono-
cytogenes, with lauric (C12:0), linoleic (C18:2), and
linolenic (C18:3) acids exerting the strongest bacteri-
cidal activity. These fatty acids also decreased the
ability of L. monocytogenes to invade mammalian
cells.
0056Antibiotic compounds, bacteriocins, produced by
lactic acid bacteria are effective against Listeria spp.
Among these are nisin produced by Lactococcus
tbl0006 Table 6 Typical generation times for Listeria monocytogenes
growing in various foods at 7
C
Food Generation time (h)
Raw milk
Naturally infected 16
Experimentally infected 19
Pasteurized milk 13
Ultra heat-treated (UHT) milk 12
Broccoli 13
Cabbage 24
Carrots 14
Radish 13
Beef 15
Lamb 9
Pork 13
Chicken 11
Chicken broth 10
Eggs, pasteurized whole 17
Crabmeat 12
Shrimp 14
Surimi 12
White fish 15
Tryptic soy broth 13
3570
LISTERIA
/Properties and Occurrence
lactis, pediocins of Pediococcus acidilacti and P. pen-
tosaceus, as well as several other compounds secreted
by other lactic acid bacteria.
0057 Some preservatives commonly used in the food
industry inhibit and/or inactivate L. monocytogenes.
These include sorbic acid, sodium benzoate, and
sodium propionate. The levels of nitrite (120 p.p.m.)
and salt (3%) used in the meat industry reduced
numbers of the organism during production of saus-
age lasting 21 days, but surviving cells were detect-
able. Populations of L. monocytogenes can be reduced
to below detection limits by a variety of liquid smoke
products.
0058 Among other compounds that inhibit this bacter-
ium are lauric (C12:0), linoleic (C18:2), and linolenic
(C18:3) acids, monolaurin, organic acids, and high-
molecular-weight sodium polyphosphates. Extracts
of numerous herbs and spices also have the ability to
inhibit growth of L. monocytogenes, including rose-
mary extract and cinnamon extract.
0059 It is noteworthy that the efficacy of these antilister-
ial compounds in food is dependent on many factors,
such as the food matrix, pH, and a
w
.
Interaction with Other Microorganisms
0060 Growth of L. monocytogenes is inhibited in the pres-
ence of lactic acid bacteria, including Lactococcus
lactis, L. cremoris, Leuconostoc gelidum, and Lacto-
bacillus bulgaricus. The inhibition is partly due to
production of bacteriocins by the lactic acid bacteria
but there is also a substantial pH effect. In most
instances growth of the pathogen is completely
inhibited when the pH drops below 4.75.
0061Interactions between Listeria monocytogenes and
other organisms have been described. Coinoculation
of milk or broth with L. monocytogenes and Pseudo-
monas spp. resulted in either slight retardation of
growth of the pathogen or no effect, depending on
strains and temperature of incubation. However, a
significant stimulation of growth of L. monocyto-
genes in milks that had previously supported growth
of Pseudomonas spp. has been reported. This effect
on growth is due to the production of proteases by the
pseudomonads. Accelerated growth of L. monocyto-
genes in whey cultured with Penicillium camembertii
compared with uncultured whey has been reported.
Foodborne Outbreaks of Listeriosis
0062Several major outbreaks of listeriosis have been de-
scribed where food was shown to be, or was strongly
implicated as, the source of infection (Table 7).
Several other sporadic outbreaks have occurred
where food has been the suspected vehicle for infec-
tion. In most foodborne cases, the foodstuff was
either unprocessed or inadequately processed. Some
of the major foodborne outbreaks of listeriosis are
discussed.
tbl0007 Table 7 Notable outbreaks of listeriosis associated with foods
Year Location/country Approximate
no. of cases
(deaths)
Possible vehicle
for transmission
Listeria
monocytogenes
serotype
1979 Boston, USA 20 (5) Raw celery, tomatoes, lettuce 4b
1980 New Zealand 29 (9) Shellfish, raw fish 1/2b
1981 Maritime Provinces, Canada 41 (17) Coleslaw 4b
1983 Massachusetts, USA 49 (14) Pasteurized milk 4b
1985 California, USA 142 (48) Jalisco cheese 4b
1983–87 Switzerland 122 (31) Raw milk cheese 4b
1986–87 Philadelphia, USA 36 (16) Icecream, salami Several
1989 Connecticut, USA 9 (1) Shrimp 4b
1987–89 UK 355 (94) Pa
ˆ
te
´
4b
1990 West Australia 11 (6) Pa
ˆ
te
´
1/2a
1991 Tasmania, Australia 4 (0) Smoked mussels 1/2
1992 New Zealand 4 (2) Smoked mussels 1/2
1992 France 279 (83) Pork tongue in jelly 4b
1993 France 39 (12) Pork rillettes (pa
ˆ
te
´
)4b
1993 Italy 23 (0) Rice salad 1/2b
1993–94 Texas, USA 8 (1) Frozen mixed vegetables 4b
1994 USA 45 (0) Chocolate milk 1/2b
1994–95 Sweden 6 (1) Cold-smoked rainbow trout 4b
1995 France 20 (4) Raw milk soft cheese 4b
1998–99 USA 100 (21) Hot dog sausages 4b
1999–2000 France 6 (2) Rillettes, langoustines 4b
1999–2000 France 32 (10) Pork tongue in jelly 4b
2000 USA 29 (4) Processed turkey ?
LISTERIA
/Properties and Occurrence 3571
Maritime Provinces, Canada, 1981
0063 This outbreak was traced to contaminated coleslaw.
L. monocytogenes of the same serotypes were isol-
ated from patients, coleslaw from the refrigerator of
an infected person, and unopened packs of the prod-
uct from the same manufacturer. The coleslaw had
been made from cabbages grown on land fertilized by
sheep manure from a flock known to be infected with
listeriosis.
0064 An outbreak of the illness at a hospital in Boston in
1979 may also have been due to the consumption of
contaminated raw vegetables.
Massachusetts, 1983
0065 A case-control study showed that this outbreak was
strongly associated with the consumption of pasteur-
ized milk. The organism was not isolated from the
implicated milk, but cases of bovine listeriosis had
occurred in herds supplying milk to the creamery
involved. This led to the suggestion that L. mono-
cytogenes could survive pasteurization due to its
intracellular location in somatic cells. Plant inspec-
tion revealed no obvious faults that would have
resulted in inadequate heat treatment or postpasteur-
ization contamination. However, the fact that the
pasteurizer was not located adjacent to the packaging
room could have been a contributing factor in the
contamination. A few facts are difficult to explain.
For instance, the strain isolated from the raw milk
was a different phage type from the epidemic isolate.
Also, only consumption of 2% fat and whole milk
was associated with infection and not skim milk,
which was processed on the same day, with the same
equipment and milk supply.
California, 1985
0066 An outbreak of listeriosis in California between Janu-
ary and August 1985 resulted in 48 deaths. The most
likely source of infection was Mexican-style cheese
from one manufacturer. L. monocytogenes serotype
4b was isolated from the cheese and processing plant.
Phosphatase tests on the cheese showed that pasteur-
ization was inadequate or the product was contamin-
ated with raw milk. Inspection of the plant revealed
several defects. There were holes in the pasteurizer
plates, a cleaning-in-place line connected raw milk
supply to the cheese vat, and the capacity of the
pasteurizer was exceeded. This led to the conclusion
that raw milk was the cause of the outbreak.
Vaud, Switzerland, 1983–87
0067 Only sporadic cases of listeriosis had been encoun-
tered in the Swiss canton of Vaud prior to 1983. The
number of cases increased dramatically in the winter
of that year. In the following years a similar seasonal
peak in cases was recorded. Cheeses were found to
harbor L. monocytogenes, including a particular type
of soft cheese exclusively produced in the winter and
mainly eaten in the region of the listeriosis outbreak.
The sero- and phage type of the strains isolated from
this cheese, Vacherin Mont d’Or, were the same as the
epidemic isolates. The cheese was made from raw
milk, although L. monocytogenes was rarely isolated
from the milk supply (< 1% of samples positive). In
1987 the product was recalled and production
stopped. After introduction of these measures. no
cases of listeriosis were recorded in the area in 1988.
France, 1992
0068An outbreak of listeriosis in France in 1992 was
reported to be responsible for 279 cases, including
22 abortions and 63 deaths. It was attributed to the
consumption of pork tongue in aspic, but some other
cases may have been caused by other delicatessen
products that had become cross-contaminated. The
epidemic strain was isolated from 220 food products
at 30 retail stores where the pork tongue was sold.
France, 1995
0069An outbreak of listeriosis in France in 1995 was
traced to the consumption of Brie de Meaux cheese
made from unpasteurized milk. The outbreak in-
volved 20 cases and four deaths. Further cases were
prevented by instigating disinfection and other con-
trol measures at the cheese plant.
USA 1998–2000
0070Between 2 August 1998 and 8 February 1999, the
Centers for Disease Control and Prevention (CDC)
reported an outbreak of listeriosis involving a rare
strain of L. monocytogenes, serotype 4b, involving
approximately 100 illnesses in 22 states covering
almost every region of the contiguous USA. This out-
break was responsible for 21 deaths (15 adults and six
miscarriages/stillbirths). CDC and state and local
health departments identified the vehicle for trans-
mission as hot dogs and possibly deli meats produced
under many brand names by one manufacturer, Bil
Mar Foods. On 22 December 1998, the manufacturer
issued a voluntarily recall of specific production lots
of hot dogs and deli meats that might be contamin-
ated and the number of illnesses caused by the out-
break strain decreased dramatically following the
recall. The direct cost of recalling the meats was $76
million, but that did not include the resulting loss of
sales. Meat sales dropped about $200 million in the 6
months after the recall. The outbreak strain of
L. monocytogenes was isolated from an opened and
3572
LISTERIA
/Properties and Occurrence