598 Modern Food Microbiology
in cold environments even though the growth of serovars 1/2a, 1/2b, and 4b was lower at around 3.0
◦ C
than those with 0I antigens. The maximum growth temperature for listeriae is around 45
◦ C.
Effect of a w
Using brain heart infusion (BHI) broth, three humectants, and 30
◦ C incubation, the minimum a w
that permitted growth of serotypes 1, 3a, and 4b of L. monocytogenes revealed the following: 0.90
with glycerol, 0.93 with sucrose, and 0.92 with NaCl.
34 In another study using trypticase soy broth
base at a pH of 6.8 and 30
◦ C incubation, the minimum a w that permitted growth was 0.92 with sucrose
as humectant.
103 In view of these findings, L. monocytogenes is second only to the staphylococci as a
foodborne pathogen in being able to grow at a w values <0.93.
DISTRIBUTION
The Environment
The listeriae are widely distributed in nature and can be found in decaying vegetation and in soils,
animal feces, sewage, silage, and water. In general, listeriae may be expected to exist where the lactic
acid bacteria, Brochothrix, and some coryneform bacteria occur. Their association with certain dairy
products and silage is well known, as is the association with these products of some other lactic acid
producers. In a study of gull feces, rooks, and silage in Scotland, gulls feeding at sewage works had a
higher rate of carriage than those elsewhere, and fecal samples from rooks generally had low numbers
of listeriae.
39 L. monocytogenes and L. innocua were most often found with only one sample containing
L. seeligeri. In the same study, L. monocytogenes and L. innocua were found in 44% of moldy silage
samples and in 22.2% of big bale silage. In Denmark, 15% of the 75 silage samples were positive
for L. monocytogenes, as was 52% of the 75 fecal samples from cows.
118 The organism was found in
silage with a pH above and below 4.5. L. monocytogenes was isolated from 8.4 to 44% of samples
taken from grain fields, pastures, mud, animal feces, wildlife feeding grounds, and related sources.
129
Its survival in moist soils for 295 days and beyond has been demonstrated.
130 From California coastal
waters, 62% of 37 samples of freshwater or low-salinity water and 17.4% of 46 sediment samples
were positive for L. monocytogenes, but none could be recovered from 35 oyster samples.
18 Some
of the ways in which L. monocytogenes is disseminated throughout the environment, along with the
many sources of the organism to humans, are illustrated in Figure 25–4.
Foods and Humans
It is well established that any fresh food product of animal or plant origin may harbor varying
numbers of L. monocytogenes. In general, the organism has been found in raw milk; soft cheeses; fresh
and frozen meat, poultry, and seafood products; and on fruits and vegetable products. Its prevalence in
milk and dairy products has received much attention because of early outbreaks. In bulk tank raw milk
from 260 farms in Scotland examined over a year, 25 of the 160 had positive samples usually only once,
but 7 were positive three or more times, usually with <1 cfu/ml with the single highest being 35 cfu/ml.
40
Of 5779 retail foods examined in The Netherlands in 1988, 3.0% were positive for this organism at
≥10/g. The lowest prevalence was in ice cream, in which only 0.2% of 649 samples were positive,
and the highest was in fresh meat, with 7.5% of 416 samples being positive. Also, in The Netherlands,
4.6% of 929 samples of soft cheeses made from raw milk were positive for L. monocytogenes for a
in cold environments even though the growth of serovars 1/2a, 1/2b, and 4b was lower at around 3.0
◦ C
than those with 0I antigens. The maximum growth temperature for listeriae is around 45
◦ C.
Effect of a w
Using brain heart infusion (BHI) broth, three humectants, and 30
◦ C incubation, the minimum a w
that permitted growth of serotypes 1, 3a, and 4b of L. monocytogenes revealed the following: 0.90
with glycerol, 0.93 with sucrose, and 0.92 with NaCl.
34 In another study using trypticase soy broth
base at a pH of 6.8 and 30
◦ C incubation, the minimum a w that permitted growth was 0.92 with sucrose
as humectant.
103 In view of these findings, L. monocytogenes is second only to the staphylococci as a
foodborne pathogen in being able to grow at a w values <0.93.
DISTRIBUTION
The Environment
The listeriae are widely distributed in nature and can be found in decaying vegetation and in soils,
animal feces, sewage, silage, and water. In general, listeriae may be expected to exist where the lactic
acid bacteria, Brochothrix, and some coryneform bacteria occur. Their association with certain dairy
products and silage is well known, as is the association with these products of some other lactic acid
producers. In a study of gull feces, rooks, and silage in Scotland, gulls feeding at sewage works had a
higher rate of carriage than those elsewhere, and fecal samples from rooks generally had low numbers
of listeriae.
39 L. monocytogenes and L. innocua were most often found with only one sample containing
L. seeligeri. In the same study, L. monocytogenes and L. innocua were found in 44% of moldy silage
samples and in 22.2% of big bale silage. In Denmark, 15% of the 75 silage samples were positive
for L. monocytogenes, as was 52% of the 75 fecal samples from cows.
118 The organism was found in
silage with a pH above and below 4.5. L. monocytogenes was isolated from 8.4 to 44% of samples
taken from grain fields, pastures, mud, animal feces, wildlife feeding grounds, and related sources.
129
Its survival in moist soils for 295 days and beyond has been demonstrated.
130 From California coastal
waters, 62% of 37 samples of freshwater or low-salinity water and 17.4% of 46 sediment samples
were positive for L. monocytogenes, but none could be recovered from 35 oyster samples.
18 Some
of the ways in which L. monocytogenes is disseminated throughout the environment, along with the
many sources of the organism to humans, are illustrated in Figure 25–4.
Foods and Humans
It is well established that any fresh food product of animal or plant origin may harbor varying
numbers of L. monocytogenes. In general, the organism has been found in raw milk; soft cheeses; fresh
and frozen meat, poultry, and seafood products; and on fruits and vegetable products. Its prevalence in
milk and dairy products has received much attention because of early outbreaks. In bulk tank raw milk
from 260 farms in Scotland examined over a year, 25 of the 160 had positive samples usually only once,
but 7 were positive three or more times, usually with <1 cfu/ml with the single highest being 35 cfu/ml.
40
Of 5779 retail foods examined in The Netherlands in 1988, 3.0% were positive for this organism at
≥10/g. The lowest prevalence was in ice cream, in which only 0.2% of 649 samples were positive,
and the highest was in fresh meat, with 7.5% of 416 samples being positive. Also, in The Netherlands,
4.6% of 929 samples of soft cheeses made from raw milk were positive for L. monocytogenes for a