Listeria monocytogenes:

An OverviewCatherine N. Cutter

Department of Food Science

Penn State University

©2006 Department of Food Science - College of Agricultural Sciences at Penn State UniversityPenn State is committed to affirmative action, equal opportunity, and the diversity of its workforce.

This publication is available in alternative media on request.

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

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; 65-100F)

Temperatures <32F moderately inactivate LM LM can survive freezing

Growth Temperatures: Refrigeration

Growth of Listeria monocytogenes CFA 433 in chicken broth when

incubated at 47.7. 38.3, 34.7, and 33.4oF. Adapted from S.J. Walker et al. (1990).

4

5

6

7

8

9

0 10 20 30 40 50

Time (days)

log1

0 cf

u/m

l

47.7F

38.3F

34.7F

33.4F

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 0.79-

0.86 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

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