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| The Bug Bible > Listeria monocytogenes
Listeria monocytogenes Description
Listeria monocytogenes are facultative anaerobic non-sporing short Gram-positive motile rods that are catalase positive. Cells grown in liquid media have a characteristic tumbling motility when observed under the microscope. They are widespread in nature, being commonly found in the environment and carried by many species of both domestic and wild animals. L. monocytogenes grows under refrigeration temperatures from below 1°C up to 44°C. Grow below 1ºC is slow. It grows at pH values of between 4.4 and 9.6 and the minimum water activity value for growth is 0.90.although the organism can survive for extended periods at lower water activities. L. monocytogenes is easily destroyed by heat and food poisoning outbreaks are relatively rare with most listeriosis cases in Australia occurring as sporadic illness
Characteristics of Illness
Listeriosis is the name of the general group of disorders caused by L. monocytogenes. It is an infection caused by the ingestion of live cells.
Infective dose: The infective dose of L. monocytogenes is unknown but is believed to vary with the strain and susceptibility of the victim. In some cases the infective dose was found to be very low, 100 to 1000 cells.
Onset of symptoms: The onset time can range from 1 to 90 days, although the average is usually 30 days.
Symptoms: The symptoms of listeriosis are variable and are dependant on the susceptibility of the person affected. Some people experience flu-like symptoms such as fever, convulsions, chills, backache, headache, diarrhoea and vomiting. Susceptible people may experience pyrexia, meningitis and septicaemia. In immune-compromised individuals and during pregnancy the disease can be more serious. Infection in late pregnancy can result in spontaneous abortion, foetal death and stillbirth. The mother may be asymptomatic.
Duration of Illness: Unknown
Diagnosis of human illness
Listeriosis can only be positively diagnosed by culturing the organism from blood, cerebrospinal fluid, or stool samples (although the latter is difficult and of limited value as many healthy people will have Listeria species present in their stools).
Complications
Most healthy persons probably show no symptoms. It can nevertheless be very serious in some cases. When listeria meningitis occurs, the overall mortality may be as high as 70%; from septicaemia 50%, from perinatal/neonatal infections greater than 80% in the foetus. In infections during pregnancy, the mother usually survives.
Susceptible individuals
The main people at risk of developing illnesses caused by L. monocytogenes are:
- Pregnant women/foetus - perinatal and neonatal infections.
- Persons immune-compromised by corticosteroids, anti-cancer drugs, graft suppression therapy, AIDS.
- Cancer patients - leukemic patients particularly.
- Patients with an underlying condition as such diabetic, cirrhotic, asthmatic, and ulcerative colitis patients
- the elderly.
The healthy population is also at risk if the food is highly contaminated with the micro-organism.
Associated foods
L. monocytogenes has been associated with such foods as unpasteurised and pasteurised milk, raw vegetables, coleslaw, soft cheeses, smoked mussels, smoked salmon and trout and meat pâtés.
Food analysis
Conventional cultural methods can be used to detect L. monocytogenes in food. These methods take around one week to give a presumptive result, with confirmation available after 10-12 days. Alternatively, rapid methods are available that give a presumptive within 48 to 72 hours.
Outbreaks
In Australia, there have been two reported outbreaks of Listeriosis. In 1990 six stillborns were reported. The same subtype of L. monocytogenes was isolated from the stillbirth autopsies and a particular brand of paté.
An episode occurred in 1991 where three people became ill after eating smoked mussels. The mussels were tested and they were found to contain 107 cells per gram.
Sporadic cases occur yearly, with round 40 being reported each year. Overseas there have been much larger outbreaks of listeriosis. In the USA, 86 to 314 cases of listeriosis were linked to the consumption of a Mexican-style fresh cheese. There was a 30% mortality rate. The L. monocytogenes was traced to the cheese factory where raw milk was added to the pasteurised cheese as it was believed that it enhanced the flavour of the cheese.
In France in 1992, 270 cases of listeriosis were reported, resulting in 63 deaths and 22 abortions. The illness was linked to the consumption of pork tongue in jelly.
Prevention
Keep food out of the Temperature Danger Zone (5°C to 60°C).
Ensure food is reheated to 75°C.
Store raw and cooked food separately.
Prevent cross contamination.
Use clean utensils and equipment to prepare food.
For ready to eat foods strict temperature control is important. Vegetables should be stored at <4ºC. A shelf life of no more than 7 days is recommended. The UK Guidelines for Cook Chill products recommends storage for no longer than 3 days at <4ºC
Practise good personal hygiene making sure that hands are washed regularly with soap, particularly after going to the toilet and after handling raw foods.
Wipe up meat and vegetable residue with disposable paper towels.
Ensure that kitchen sponges and clothes are disinfected and replaced regularly.
Susceptible individuals should avoid high risk foods such as pates, soft cheeses, unpasteurised dairy products and uncooked seafood.
Further Information
FDA Bad Bug Book
Literature:
Hocking, A.D. et al. (2003). Foodborne Microorganisms of Public Health Significance. 6th ed. North Sydney. AIFST NSW Branch Food Microbiology Group.
Doyle, M.P. (1989). Foodborne Bacterial Pathogens. Marcel Dekker; New York. |
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