Fact sheet on Listeria spp. and Listeria monocytogenes

General information
Listeria monocytogenes (L. monocytogenes) is the most important member of the genus Listeria spp., as L. monocytogenes can cause severe foodborne illness and the number of reported listeriosis cases has tended to increase in recent years. These bacteria play a major role in the microbiological quality control of many perishable foods.
In food businesses, Listeria can colonize certain ecological niches and persist there for long periods. They are often found in damp areas, in slimy deposits or biofilms. L. monocytogenes is cold-tolerant and can sometimes even grow at refrigeration temperatures.

Characteristics

• Rod-shaped bacteria
• Gram-positive
• Oxidase-negative
• Catalase-positive

Origin / Occurrence

• Widespread in our environment (e.g. soil, water, plants and animals)
• Because of their wide distribution, Listeria and L. monocytogenes are regularly found in many foods (especially those containing raw animal and plant products)

Significance

• L. monocytogenes are feared foodborne pathogens. Particularly at risk are pregnant women, infants, young children, older people and immunocompromised individuals. Complications are relatively rare in healthy people
• Illnesses occur in various forms. In addition to gastrointestinal infections, other organs can be affected. Invasive listeriosis (e.g. meningitis, sepsis, …) is a serious infectious disease that can be fatal
• Because Listeria and L. monocytogenes regularly occur in our environment and in many raw foods, their complete absence in many foods is difficult to guarantee
• L. monocytogenes can multiply even at refrigeration temperatures. Therefore, perishable foods with shelf lives of more than one week should be particularly well protected against contamination
• Due to its properties, L. monocytogenes plays an important role in the microbiological quality control of perishable foods. Environmental testing is highly relevant in addition to product testing
• Testing for Listeria spp. is a good indicator for the potential presence of L. monocytogenes

Clinical features

• Minimal infectious dose: generally from 10³ to 10⁴ CFU/g; in high-risk groups, e.g. pregnant women, already from 10 CFU/g
• Incubation period: 3 to 90 days
• Duration of illness: normally a few days; much longer in case of complications
• Symptoms: classic gastrointestinal illness with fever, muscle pain, nausea, diarrhoea; complications possible such as meningitis (inflammation of the meninges), sepsis, miscarriages and preterm births in pregnant women
• Relatively high mortality rate, especially in immunocompromised persons

Examples of affected foods

• raw meat (e.g. minced meat) and poultry
• meat and sausage products (especially sliced cold cuts, head cheese), raw sausages
• fish products (vacuum-packed smoked salmon and trout, shellfish)
• raw milk / raw milk products, soft cheeses, acid-curd cheeses
• packaged, pre-cut leafy salads and vegetables
• delicatessen salads (e.g. coleslaw)
• surface water

Growth conditions

• Temperature:
  • Optimum: +30 °C to +37 °C
  • Minimum: 0 °C
  • Generally no growth above +50 °C
• pH value: growth at pH 4.5 to 9.0
• a_w-value: growth up to a water activity of 0.93 or higher
• Salt tolerance: max. 10%
• Oxygen requirement: facultatively anaerobic (microaerophilic)
• Only low nutrient requirements

Inactivation by heating

• At +72 °C for at least 2 minutes of exposure time (Note: check core temperature)

1. Check the execution of cleaning and disinfection:
   1. Hygiene plan
   2. Execution of cleaning and disinfection, e.g. dosing of the disinfectant, contact time, use of suitable and clean materials and cleaning cloths
2. Avoid moisture residues and biofilms. Ensure thorough drying of equipment, devices and surfaces
3. Train staff about potential errors and corrective measures
4. Repeat cleaning and disinfection
5. Verify success by visual on-site inspections, environmental testing and follow-up testing

1. Review workplace organization in the facility
   1. Separate work areas, equipment and utensils for raw and processed foods
   2. Separation of "clean" and "unclean" areas and activities
   3. Train staff about potential errors and corrective measures
2. Verify success by visual on-site inspections and follow-up testing

1. Check processes, production protocols and documentation
2. Most bacteria are inactivated at a core temperature of +72 °C for at least 2 minutes. Exceptions include heat-resistant spores and toxins. In these cases significantly higher temperatures and exposure times are required. Further information can be found, for example, in the respective fact sheets
3. Verify the correctness of processes and temperatures
   1. In general, hot-holding temperatures above +60 °C are required
   2. Rapid cooling: after 3 hours a core temperature of at least +7 °C must be reached (special caution required for large quantities)
4. Verify success by follow-up testing

1. Request traceability of the cold chain during transport or from the supplier
2. Check incoming and storage temperatures
3. Monitor refrigeration temperatures / cold chains during processing, transport and sale
4. If over-aging (or excessively long shelf life) is suspected, carry out appropriate storage tests
5. Verify success by follow-up testing

1. Check incoming products at delivery for e.g. incoming/transport temperature, shelf life, packaging, deviations or abnormalities
2. This also applies to goods prior to use in production. In case of doubt do not use the food and consult a supervisor
3. Check microbiological specifications. If necessary, arrange laboratory testing of suspicious products or request results from the supplier
4. Verify success by follow-up testing

1. Train staff about potential errors and corrective measures
2. Regular cleaning and disinfection of hands and regular glove changes
3. Regular change of work clothing. Ensure work clothing is kept clean
4. Train employees on infection control requirements
5. Verify success by visual on-site inspections and follow-up testing

1. Check processes, environmental conditions, production protocols and documentation
2. Verify success by follow-up testing