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 Penn 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; F)  Temperatures <32F moderately inactivate LM  LM can survive freezing

Growth Temperatures: Refrigeration

Acidity  Typical pH range is 5.0 to 9.6  Optimum =neutral conditions ~  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 (a w )  LM grows well at a w of >0.95; can multiply at a w of 0.90  Some LM can survive at a w of <0.90  LM was isolated from salami with a w of  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 (48 deaths)  Hotdogs and deli meats – (20 deaths)  Ready-to-eat turkey and chicken products – 2002 (10 deaths)

Listeriosis: Outbreaks  Largest Outbreak  Bil Mar Foods Outbreak,  101 cases  21 deaths  Rare strain of L. monocytogenes 4b  Recalled 35 million pounds of hot dogs and deli meats

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