Listeria monocytogenes

family

Listeriaceae 

genus

Listeria, with L. monocytogenes  

being the only species pathogenic  

for humans among the six Listeria species

gram positive or negative

positive

shape 

motile rod 

growth temp

Remarkably, it grows in chilled foods, with a minimum growth temperature (Tmin) of 0°C. It can grow at refrigeration temperatures

minimum water acitivty

Salt-tolerant, with a minimum water activity (aw,min) for growth of 0.92 

type of organism

facultative anaerobe- does not require 02 but grows better with it. 

Vegetative Cells (Non-Sporeformers) 

Ubiquitous pathogen

survival

•    It has the potential for outgrowth in refrigerators, as a significant proportion of domestic refrigerators are kept above 5°C. 

•   Modified atmosphere packaging and vacuum packaging do not prevent its growth. 

• Survivives in dry circumstances. 

disease

Listeriosis: Can cause two forms of illness 

• Mild intestinal illness 

• Severe illness 

symtons

Mild intestinal illness: Presents as gastro-enteritis or a mild flu-like illness  

 

Severe illness: Can lead to serious conditions like stillbirth (in fetuses), septicemia, and meningitis 

incubation time

Mild- less than 1 day to 48 hours 

Severe- less than 1 day up to 3 months 

duration ilness

Mild- illness lasts 1-3 days 

D/R

The dose-response relation is not clear

transmssion and sources

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

Risk in ready-to-eat meals preserved by refrigeration. 

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

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

    ◦ 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

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 

D value

The D-value in milk is 1-2 seconds at 71.7°C 

factors affecting D value

D-values are lower at higher temperatures (faster inactivation)

Z value

value of 5°C, indicating that the D-value (decimal reduction time) decreases by one log for every 5°C increase in temperature 

mortality rate

High about 16-30%