Listeria can grow at fridge temperatures. And it’s nasty. A cause of serious food poisoning causing flu-like symptoms, fever, diarrhoea and a risk of miscarriage in pregnant women. It is fatal in 20% of cases. Listeria is particularly dangerous as it can multiply at fridge temperatures, albeit slowly.While disease caused by this organism is uncommon, the clinical consequences are often serious. Two forms of disease are now recognised; a serious invasive disease and a non-invasive gastroenteritis. It grows at refrigeration temperatures in the presence or absence of air.
Growth and Control
Temperature: Optimum 37°C, range -1.5 to 45 °C.
pH: Optimum 7.0, range 4.4-9.4.
Atmosphere: Grows optimally under microaerophilic conditions but grows well both aerobically and anaerobically. Can grow in relatively high (e.g. 30%) CO2, but is inhibited under 100% CO2. Growth was not retarded by a 5-10% CO2 atmosphere.
Water Activity: Minimum aw permitting growth = 0.92 (≡11.5 % NaCl). Will grow in media containing up to 10% NaCl.
Temperature: Survives freezing very well.
Atmosphere: Not influenced by atmosphere.
Inactivation (CCPs and Hurdles)
Temperature: Rapidly inactivated at temperatures above 70°C. D time at 50°C can be in the order of hours, at 60°C 5-10 min, 70°C approximately 10 sec.
pH: Inactivated at pH values less than 4.4 at rates depending on the acidulant and temperature. Organic acids, such as acetic, are more effective than mineral acids (e.g. hydrochloric). Inactivation proceeds faster at higher temperatures.
Water Activity: Can remain viable in dry environments for long periods.
Preservatives: Inactivated on vegetables by lysozyme (100 mg/kg), 0.2% sodium benzoate at pH 5, 0.25- 0.3% sodium propionate (pH 5, less effective at lower temperatures), and 0.2-0.3% potassium sorbate (pH 5.0).
The addition of nitrite to salami-type meat batter minimally affected survival of the organism at 37°C (pH was the primary factor). The use of appropriate starter cultures results in the elimination of the organism from salami.
In other meats at around pH 6-6.3, nitrite (70-140ppm) did retard growth, and sodium ascorbate (0.042%) in combination with the nitrite retarded growth further. Ascorbate had no effect in the absence of nitrite.
Lactate and ALTA 2341 (shelf life extender) lengthened lag times in poultry but effectiveness decreased as temperature increased. Inhibited by 100 ppm monolaurin or 1000 ppm eugenol.
See here for guidance.
Gaseous acetic acid did not disinfect all samples of inoculated mung beans.
Sanitisers/disinfectants (aldehydes, alcohols, ethanol/phenols, substituted phenols, disochlorine, quaternary ammonium compounds (QACs)) are generally effective in the absence of organic matter.
There are two types of disease associated with the organism; invasive and non-invasive. The invasive disease normally occurs in people with weakened immune systems, while the non-invasive disease can occur in anyone if a high number of L. monocytogenes cells is consumed.
Incubation: Invasive: 1-90 days, mean 30 days. Non invasive: 11 hours to 7 days, median 18 hours.
Symptoms: Invasive: Include ‘flu’-like symptoms (e.g. fever, headache), diarrhoea, vomiting, meningitis, septicaemia, spontaneous abortion. Non-invasive: Diarrhoea, fever, muscle pain, headache, and less frequently with abdominal cramps and vomiting. Attack rate reported to be 74%.
Condition: Invasive: Listeriosis. A mortality rate of approximately 30% is associated with the disease. Hospitalisation rate: 92%. Non-invasive: Has been termed non-invasive febrile gastroenteritis.
Toxins: No toxins are produced in foods.
At Risk Groups: Invasive: Those at risk include pregnant women and their foetuses, new born children, the elderly and those with compromised immune systems, e.g. AIDS patients. Non-invasive: Will affect anyone consuming high numbers of cells.
Long Term Effects: In one outbreak neurological problems (cranial nerve palsies) developed in 30% of the survivors of meningitis. Pre-term infants may suffer from excess fluid in the brain, requiring surgery, and partial paralysis.
Dose: Invasive: The estimate of the number of cells that need to be ingested to cause disease is open to debate. A general consensus seems to be that 100- 1,000 cells are normally required. Non-invasive: Outbreaks have been attributed to foods containing >105 cells/g, and in one case the median consumption of cells was estimated to be 1011.
Treatment: L. monocytogenes is susceptible to a number of antibiotics, but penicillin and ampicillin optionally with an aminoglycoside (e.g. gentamicin) is considered to be the combination of choice.
Reservoirs / Sources
Human: L. monocytogenes is carried asymptomatically in the faeces of 2-6% of the population. Person-to-person spread (other than mother to foetus) not often recorded but has been recognised. Up to 30% of case contacts may carry the organism. Is shed in high numbers (≥ 104/g) in the faeces of infected people.
Animal: Can cause disease in animals, and veterinarians were originally considered to be the at risk group. Listeria present in animal faeces can contaminate red meat. Improperly made silage can be a source of domestic animal infection.
Food: Should be considered as potentially present in all raw foods and ingredients. May be present in cooked foods as a result of post-cooking contamination. Risk posed is likely to be greatest in ready-to-eat cooked foods with long shelf lives. Has been isolated from a wide variety of ready-to-eat and raw foods but is generally considered to be usually present in low numbers (<10/g) on foods, although it has been detected at numbers far in excess of this.
Environment: Is widespread in the environment including soil, vegetation, water and sewage. Has been isolated from toothbrushes and other domestic environments.
Transmission Routes: One study estimates that 1/3 of cases are foodborne. Other reports describe foodborne transmission as the primary source of human infections. Alternative routes include infections acquired in hospital and occupational exposure (e.g. vets).
Plague and Pestilence
Outbreaks: Most cases of invasive listeriosis are sporadic, but large outbreaks have occurred. Outbreaks feature prominently in the media because of the associated high case fatality rate.
Non-invasive listeriosis has been recognised because of the outbreaks of disease that have resulted from the ingestion of large numbers of the organism.
Epidemiological studies: Factors reported include consuming unpasteurised milk, uncooked hot dogs, undercooked chicken, soft cheeses and foods purchased from delicatessen counters.