Listeria is commonly found in raw foods such as ground meat, raw milk, fish, and salads. They are extremely adaptable bacteria that can be found all over the world and require few nutrients to survive. The resilience of these bacteria is demonstrated by the fact that they can survive even in the absence of air in a vacuum and at refrigerator and freezer temperatures. Only sufficient heating of at least 70 degrees Celsius kills Listeria.
What are listeria?
Listeria are undemanding bacteria that can survive in almost any environmental conditions. They are gram-positive, anaerobic, and endospore-forming bacteria. The 1.5 µm rod and filamentous bacteria actively move by swimming. They are ubiquitous in nature because the hardy bacteria can multiply even in nutrient-poor substrates such as condensation and puddles. Extremely cold– and heat-tolerant, Listeria does not die until temperatures reach 70 degrees Celsius and is commonly found in raw meat products and raw milk products. There are different dangerous Listeria species. The most common species is Listeria monocytogenes. The Federal Institute for Consumer Health Protection and Veterinary Medicine recommends a limit of 100 germs per milliliter or gram of food, as there is no legally prescribed limit in Germany. The minimum infectious dose (MID) at which listeria causes illness is still unknown. However, medical experts assume that a very high bacterial concentration must be present.
Occurrence, distribution, and characteristics
Because listeria are very undemanding and hardy, they are found almost everywhere in nature. They are heat and cold tolerant and can survive even in the most adverse environmental conditions. Because the 1.5 µm rod- and filamentous-shaped bacteria can swim, they are commonly found in flowing water, puddles and condensation. They also feel at home on plant materials such as dead grasses and in the intestinal tract of animals and humans. About ten percent of people are carriers of Listeria and excrete it in their stool. Humans come into contact with Listeria through contaminated food, which resides primarily in raw meat products, fish, and raw dairy products. Because Listeria is ubiquitous in the environment, it can also be found on plant foods such as lettuce. In contrast to their natural occurrence in nature, listeria enter the aforementioned foods only in a roundabout way, because the contamination arises in the various production stages during food processing due to poor hygiene and insufficient heating. Listeria can get onto vegetables and salads through fertilization with animal wastewater such as manure. These bacteria also survive in vacuum-packed foods because they are able to switch between aerobic and anaerobic metabolism. This ability means that, unlike many other germs, Listeria can survive even when oxygen levels are reduced. Optimal growth rates for these bacteria occur in a temperature range between 30 and 37 degrees Celsius with a pH between 5.0 to 9.0 and a slightly elevated carbon dioxide concentration. Even under extended temperature conditions, Listeria are able to survive when competing germs have long ceased their activity. However, frying, boiling, pasteurization, and sterilization kill the germs. In sour or salty foods, reproduction is only possible with a delay or is no longer possible. Infection cannot always be prevented with certainty, because Listeria infection is possible almost everywhere. These bacteria do not only occur during the production and processing of food at the manufacturers, but also through improper storage and preparation in the home kitchen. Kitchen-related errors such as contaminated knives, slicers, pads, plates, and improper storage and preparation conditions can cause listeria to spread ubiquitously and lead to what is known as listeriosis. Washing hands before and after food preparation also helps prevent infection. There is also an increased risk of listeria in factory farming, as these unfavorable husbandry conditions produce an increased potential for infection in farm animals.People who are particularly at risk for listeria infection should avoid eating foods from the high-risk group.
Diseases and medical conditions
For people with a healthy immune system, listeria is usually harmless because it is eliminated through the intestines with bowel movements. It is estimated that 10 percent of people are infected with listeria without it becoming noticeable as long as their immune system is functioning properly. The bacteria are dangerous for people with a weakened immune system, caused for example by tumor or flu diseases, HIV or an organ transplant. Another risk group includes the elderly, infants, young children and pregnant women. Listeriosis manifests with flu-like symptoms such as fever, muscle aches, vomiting, and diarrhea. These symptoms are often accompanied by purulent inflammation of various organs, with meningitis and sepsis being the most common. Less common are inflammations of the brain (encephalitis), conjunctiva (conjunctivitis), joints (arthritis), and heart valves (endocarditis). If listeriosis is suspected, diagnosis is made using body fluids such as blood, cerebrospinal fluid, postpartum fluid, or suspect foods. Examination of a stool sample, on the other hand, is not conclusive, since many people carry listeria without becoming ill. Treatment is with high-dose antibiotics such as amoxicillin and gentamicin. If healing is not successful, cotrimoxazole is administered. Other treatment options include macrolides, chloramphenicol and vancomycin. The treatment period depends on the severity of the disease and lasts from two to six weeks. A detailed guidebook is available from the Institute for Risk Assessment that covers all the important issues surrounding foodborne infections and provides tips on how to prevent them.
