Download Listeria monocytogenes : An Overview

Listeria monocytogenes:
An Overview
Catherine N. Cutter
Department of Food Science
Penn State University
©2006 Department of Food Science - College of Agricultural Sciences at Penn State University
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Listeria monocytogenes
Listeria monocytogenes
(LM)
 Gram positive, facultative, motile, rod
 LM is naturally found in:
 Soil and vegetation (wet and dry)
 Freshly harvested grass
 Grass with higher moisture content
 Fecal material (human and animal)
 Isolated in healthy livestock (2-16%), wild
animals, human sewage
 Polluted water
 Animal feed (silage and straw)
Characteristics of LM
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Growth temperatures
Acidity
Water activity
Air requirements
Growth Temperatures
 Growth range = 30 to 113°F (-1 to 45°C)
 Optimum = 86 to 98.6°F (30 to 37°C)
 Psychrotrophic (refrigeration temperature;
<40F)
 Mesophilic (room to body temperature; 65100F)
 Temperatures <32F moderately inactivate LM
 LM can survive freezing
Growth Temperatures:
Refrigeration
9
log10 cfu/ml
8
7
6
47.7F
38.3F
5
34.7F
33.4F
4
0
10
20
30
40
50
Time (days)
Growth of Listeria monocytogenes CFA 433 in chicken broth when
incubated at 47.7. 38.3, 34.7, and 33.4 oF. Adapted from S.J. Walker et al.
(1990).
Acidity
 Typical pH range is 5.0 to 9.6
 Optimum =neutral conditions ~6.0 - 7.0
 Growth at pH of <4.3 has not been
demonstrated
 LM can survive in hard salami at pH 4.3 to 4.5
 LM can tolerate lower pH when kept at near
refrigeration temperatures (vs. room & body
temperatures)
 Most strains of LM are unlikely to grow in food
products with pH < 5.2
Water Activity &
Salt Concentration
 Water activity (aw)
 LM grows well at aw of >0.95; can multiply at
aw of 0.90
 Some LM can survive at aw of <0.90
 LM was isolated from salami with aw of 0.790.86
 Salt concentration
 Growth at 10%
 Survival at 25.5%
Air requirements
 LM is a facultative organism
 Can grow under aerobic (oxygenated
conditions)
 Can grow under reduced oxygenated
conditions (semi-anaerobic)
 Vacuum packaging provides a
facultative environment  growth
during long term refrigerated storage
Listeria monocytogenes
 Has been recognized as a foodborne pathogen
since the 1980s
 Shown to be a pathogen in over 50 mammals
 The only Listeria species pathogenic to
humans
 Intracellular pathogen - causes the food-borne
illness, listeriosis, in susceptible persons
Listeriosis: Symptoms
 Pregnant women
 Flu-like illness, fever, muscular pain,
headache or asymptomatic
 Spontaneous abortion, stillbirth, septicemia,
meningitis
 Non-pregnant adults
 Mild gastroenteritis, septicemia, meningitis
Listeriosis:
Infectious Dose
 Minimum infectious dose (MID) not
determined
 Consumption of < 1000 organisms in milk
may cause disease
 108 cells were required to cause disease
in healthy primates
Listeriosis:
Susceptible Populations
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Elderly and infants
Pregnant women
Neonates
Impaired immune system (AIDS)
Immunosuppressive therapy for
malignancy or organ transplant
 Predisposing illness (alcoholism,
diabetes, cirrhosis of the liver)
Listeriosis: Incidence
 FoodNet Data 2003
 Incidence rate of 0.33 cases/100,000
population
 Case fatality rate was highest - 17%
 Rate of hospitalization was highest - 91%
 Incidence has decreased from 1996 by 21%
 Incidence target for 2010
 0.25 cases/100,000 population
Listeriosis: Outbreaks
 Coleslaw – 1981 (9 deaths)
 Pasteurized milk – 1983 (14 deaths)
 Mexican style soft cheese - 1985 (48
deaths)
 Hotdogs and deli meats – 1998-1999 (20
deaths)
 Ready-to-eat turkey and chicken
products – 2002 (10 deaths)
Listeriosis: Outbreaks
 Largest Outbreak
 Bil Mar Foods Outbreak, 1998-1999
 101 cases
 21 deaths
 Rare strain of L. monocytogenes 4b
 Recalled 35 million pounds of hot dogs and
deli meats
Questions