FSC-342 Quiz #3: Lecture 12 - Listeria Monocytogenes

What genus does Listeria monocytogenes belong to?

Listeria; it comprises six species. L. monocytogenes is the only species of public health concern (hemolytic), and L. innocua is nonpathogenic.

What are the physical characteristics of Listeria monocytogenes?

Gram positive short rod, found singly, in pairs, or in short chains; non-spore forming; facultatively anaerobic; exhibits tumbling motility at 20–25°C but not at 37°C.

Why is Listeria monocytogenes considered a hardy organism?

It is psychrotropic (grows 0–45°C), acid tolerant (pH 4.2–9.6), salt tolerant (grows up to 10% NaCl; survives 23% brine), and thermal tolerant.

What are the US regulatory standards for Listeria monocytogenes in foods?

Zero tolerance in ready-to-eat foods.

What are the European regulatory standards for Listeria monocytogenes?

Absence in 25 g upon release from manufacturer; maximum 100 CFU/g throughout shelf life.

How is Listeria monocytogenes transmitted?

Virtually all cases are foodborne.

What is the infectious dose of Listeria monocytogenes?

Generally greater than 100 CFU/g.

What is the incubation period for listeriosis?

Can range from 3 to 70 days.

Which populations are at high risk for listeriosis?

Pregnant women, neonates, elderly, immunocompromised adults, people with chronic diseases (heart failure, diabetes, alcoholism, liver cirrhosis), and specific ethnic groups (Hispanic in some outbreaks).

What are the manifestations of listeriosis in pregnant women?

Flu-like illness in the third trimester; can lead to spontaneous abortion, fetal death, severe neonatal septicemia, and meningitis (~10% of cases).

What are the manifestations of listeriosis in predisposed adults?

Bacteremia and/or meningitis (e.g., adults with AIDS or cancer).

What are the manifestations of listeriosis in previously healthy adults?

Meningitis or mild gastroenteritis (e.g., linked to chocolate milk).

Where is Listeria monocytogenes found in the environment?

Ubiquitous: soil, water (lakes, rivers, sewage), vegetation (silage, pasture grass).

How does Listeria monocytogenes contaminate food processing facilities?

Entry via soil on shoes, clothing, raw products, human carriers; contamination often in floor drains, coolers, aerosols, and equipment residues (biofilms).

How are slaughterhouses contaminated with Listeria monocytogenes?

During stunning and dehiding; poultry operations contaminated during defeathering, chill water, and recycled wastewater.

When does contamination primarily occur in food production?

After processing: slicing, contact surfaces (conveyor belts), packaging, retail handling, and consumer refrigerators.

Which foods pose the highest risk for listeriosis?

Ready-to-eat foods such as soft cheeses, frankfurters, deli meats, and poultry products.

Which meat products are commonly contaminated by Listeria?

Cooked/ready-to-eat meats, raw cuts and roasts, raw ground meat.

Which dairy products are commonly contaminated by Listeria?

Raw milk; cheeses like Queso fresco, Cotija, Brie, Camembert; ice cream; chocolate milk. Yogurt and butter are less frequent sources.

Which fish and seafood products are commonly contaminated by Listeria?

Smoked fish (13.6%), crab (7.5%), shrimp, lobster, mussels; domestic 4.1%, imported 8.3%.

How can vegetables become contaminated by Listeria?

Contaminated raw vegetables in processing plants.