Listeria

Family Listeria

Listeriaceae

Which species of Listeria is pathogenic for humans

L. monocytogenes

Gram +/- Listeria monocytogenes

Gram positive

Shape Listeria monocytogenes

Motile rod

Growth temperature Listeria monocytogenes

• Grows in chilled foods

  • Min growth temp is 0 degrees

  • 0°C to 45°

Min water activity Listeria monocytogenes

Minimum water activity for growth = 0.92

(An)aerobic? Listeria monocytogenes

Facultative anaerobe - does not require oxygen but grows better with it

Spores? Listeria monocytogenes

Non sporeformer

Pathogen Listeria monocytogenes

Ubiquitous pathogen

Survival Listeria monocytogenes

• Has the potential to grow a lot in refrigerator, as most refrigerators are kept around 5C

• Modified atmosphere packaging and vacuum packaging do not prevent

• Survives in dry circumstances

Disease Listeria monocytogenes

Listeriosis which can cause two forms of illness

• Mild intestinal illness

• Severe illness

Symptoms of mild intestinal illness Listeria monocytogenes

• Presents as gastroenteritis or a mild flu-like illness

• Incubation: less than 1 day up to 2 days

• Lasts 1-3 days

Symptoms of severe illness Listeria monocytogenes

• Can lead to serious conditions like stillbirth, septicemia and meningitis

• Incubation: less than 1 day up to 3 months

Sources Listeria monocytogenes

• Commonly found in deli meats, soft cheese, raw milk, pate, smoked and lightly processed fish and various ready to eat foods

• Ready to eat meals preserved by refrigeration are risky

• Often associated with cooled products that have an extensive shelf life and relatively low water activity

Transmission Listeria monocytogenes

• Most vulnerable populations are YOPI (Young, Old, Pregnant, Immunocompromised)- people with weak defences.

• Cross-contamination can occur in food factories and retail settings, for instance, from contaminated ham to processing equipment.

• Can persist on surfaces; for example, it has been found in domestic environments on dish-cloths (17% positive), washing-up brushes (6% positive), and refrigerators (63% positive)

Prevention and control Listeria monocytogenes

• To prevent outgrowth during storage, control measures include maintaining pH values at or below 4.3 (e.g., in fermented sausage with pH 4.3-4.5) and water activity at or below 0.92 (e.g., in hard cheese).

• Heat treatment is effective: Pasteurization (e.g., 15 seconds at 71.7°C) is sufficient to inactivate L. monocytogenes

• Modified atmosphere packaging and vacuum
  packaging do not prevent growth

Mortality rate Listeria monocytogenes

16-30%

Optimum pH and pH resistance for Listeria monocytogenes

5-8