Bioaerosols are living organisms or substances from living organisms suspended in the air. They are responsible for allergies, sick building syndrome, and spread of bacterial and viral disease. Bioaerosols of food-production environments are of growing interest becuase of the potential for product contamination. Many studies have analyzed the air of meat and dairy production plants, while very few have focused on RTE foods. RTE foods are of concern because of the potential contamination with Listeria monocytogenes.
Listeria monocytogenes is a food borne pathogen, which is ubiquitous in the environment. L. monocytogenes is capable of survival and growth in many conditions meant to control bacteria. The unique growth characteristics combined with the ability for food contamination by a bioaerosol are a cause for concern. In addition to its unique survival abilities, this pathogen affects individuals with compromised or weakened immune system, such as pregnant women, the elderly and young. In these groups the high mortality rate of 20-40%.
The purpose of this work is to elucidate the problem of L, monocytogenes bioaerosols in food production and assess the potential of airborne contamination by L. monocytogenes to RTE meats in an experimental setting. To simulate an airborne-like condition L. monocytogenes attached to dry sterile sand and was dusted onto meat products. The RTE meat products were exposed at three inoculum quantities [1.0, 5.0 and 10.0g]. Half the samples were evaluated at day zero and the other half at day 28. All products were direct plated on chromogenic L. monocytogenes media, and then enriched in University of Vermont [UVM] broth and 4-Morpholinepropanesulfonic acid buffered Listeria enrichment broth [MOPS-BLEB]. Following the enrichment the samples were evaluated for the presence of L. monocytogenes by plating onto Modified Oxford [MOX] media and with a commercially available Polymerase Chain Reaction [PCR] system. It was found the contamination of RTE meats by airborne L. monocytogenes was not significantly different based on inoculum quantity and cold storage. There was, however, a difference in the risk based on the RTE meat product type. These results are by not mean conclusive and more research is needed to better determine what is the potential for airborne contamination to RTE meats from L. monocytogenes bioaersosols and how the problem can be prevented or controlled during production.
Listeria monocytogenes can colonize floor drains in poultry processing and further processing facilities, remaining present even after cleaning and disinfection. Therefore, during wash down, workers exercise caution to avoid spraying hoses directly into drains in an effort to prevent the escape and transfer of drain microflora to food contact surfaces. The objective of this study was to examine the extent to which an inadvertent water spray into a colonized floor drain can cause the spread of airborne Listeria. Listeria innocua was used to inoculate a polyvinyl chloride model floor drain, resulting in approximately 10[8] cells per ml of phosphate-buffered saline and 10[4] attached cells per square centimeter of inner surface. Each model drain was subjected to a 2-s spray of tap water at 68.9 kPa from a distance of 1 m. Drains were sprayed while filled and again after emptying. Airborne cells were collected by using sedimentation plates containing Listeria selective agar which were placed on the floor and walls of a contained room at incremental horizontal and vertical distances of 0.6, 1.2, 2.4, or 4.0 m from the drain. Sedimentation plates were exposed for 10 min. A mechanical sampler was used to also collect air by impaction on the surface of Listeria selective agar to determine the number of cells per liter of air. The experiment was conducted in triplicate rooms for each of four replications. L. innocua was detected on sedimentation plates on the floor as far as 4.0 m from the drain and on walls as high as 2.4 m above the floor and 4 m from the drain. A 2-s spray with a water hose into a contaminated drain can cause airborne spread of Listeria, resulting in the potential for cross-contamination of food contact surfaces, equipment, and exposed product.
After implantation of approximately 10[3] Listeria monocytogenes organisms into the lungs, mice develop an acute pneumonitis with dissemination of infection to a mediastinal lymph node [MedLN], liver, and spleen. The infections in a MedLN and spleen resolve in approximately 7 days, but the lung infection persists for a few days longer. Pneumonitis is accompanied by a lymphoproliferative response in a MedLN and spleen, and immunity to Listeria is conferred adoptively with MedLN and spleen cells but not with mesenteric lymph node cells. Although the spleen appears to be the major repository of sensitized lymphocytes, splenectomized mice combat Listeria pneumonitis as effectively as normal mice. It is concluded that the induction of immunity to lung infection with L. monocytogenes is efficient and that the cause for the rather protracted pneumonitis is due to a defect in the expression of the cell-mediated immunity effector mechanism.
Reviewed: January 2022
What is listeriosis?
Listeriosis is an infection caused by the bacteria Listeria monocytogenes. People become infected by eating foods contaminated with the bacteria. Listeria may infect many different sites in the body, such as the brain, spinal cord membranes, or the bloodstream.
Who gets listeriosis?
Anyone can get the disease, but those at highest risk for getting it are pregnant women, the elderly, people with weakened immune systems [for example, people with cancer, HIV/AIDS, or a transplant], and people with chronic liver or kidney disease, diabetes, or alcoholism. Healthy adults and children occasionally are infected with Listeria, but they rarely become seriously ill. Although most cases occur as single cases, food-borne outbreaks [when two or more people become ill from the same source] do occur.
How is listeriosis spread?
Listeria bacteria can be found in water and soil. Infected animals, even if they are not sick, may carry the bacteria, spread it, and contaminate foods. Listeria can be spread to people by several different methods. Eating food contaminated with the bacteria, such as through raw [unpasteurized] milk or contaminated vegetables, is often a source for cases. The bacteria may be passed from mother to fetus during pregnancy or directly to the newborn at the time of birth.
What are the symptoms of listeriosis?
Because listeriosis can affect many different parts of the body, the symptoms vary from mild to severe. Listeria can cause fever and diarrhea [loose stool/poop] similar to other foodborne germs, but this type of Listeria infection is rarely diagnosed. Symptoms vary in people with invasive listeriosis, meaning the bacteria has spread beyond the gut. Symptoms start 1 to 4 weeks after eating food contaminated with Listeria.
- Pregnant women: Pregnant women typically experience only fever and other flu-like symptoms, such as fatigue and muscle aches. However, infections during pregnancy can lead to miscarriage, stillbirth, premature delivery, or life-threatening infection of the newborn.
- People other than pregnant women: Symptoms can include headache, stiff neck, confusion, loss of balance, and convulsions in addition to fever and muscle aches.
How is this disease diagnosed?
Specific laboratory tests are the only way to diagnose this disease. A blood, spinal fluid, or amniotic fluid/placenta test that looks for the bacteria will be able to show if the disease is present.
What is the treatment forlisteriosis?
Listeriosis is treated with antibiotics. Ampicillin, either alone or in combination with other antibiotics, is often used.
Does past infection with listeriosis make a person immune?
Past infection does not appear to make a person immune. People can be reinfected if exposed to the Listeria bacteria again.
What can be done to prevent the spread of this disease?
Since the bacteria is widespread in nature, basic sanitary measures such as using only pasteurized dairy products, eating cooked meats, washing produce, and washing hands, kitchen work surfaces, and utensils with soap and water immediately after they have been in contact with raw meat or poultry offers the best protection.
In addition, the following recommendations are for persons at high risk, such as pregnant women and people with weakened immune systems [for example people with cancer, HIV/AIDS, or a transplant]:
- Do not eat unpasteurized [raw] milk or dairy products.
- Eat cut melon right away or refrigerate at 41ºF or colder for no more than 7 days. Throw away cut melons left at room temperature for more than 4 hours.
- Do not eat hot dogs, lunchmeats [or deli meats], unless they are reheated until steaming hot hot or heated to an internal temperature of 165ºF..
- Safely store hots dogs and lunch meat in the refrigerator, discarding opened packages of hot dogs after 1 week and opened packages of sliced deli meat no longer than 3 to 5 days. Avoid getting fluid from hot dog and lunch meat packages on other foods, utensils, and food preparation surfaces, and wash hands with soap and water after handling hot dogs and lunch meats.
- Do not eat soft cheeses such as feta, Brie, and Camembert, blue-veined cheeses, or Mexican-style cheeses such as queso blanco, queso fresco, and Panela, unless they have labels that clearly state they are made from pasteurized milk.
- Be aware that Hispanic-style cheeses made from pasteurized milk, such as queso fresco, have caused Listeria infections, most likely because they were contaminated during cheese-making.
- Do not eat refrigerated pâtés or meat spreads. Canned or shelf-stable pâtés and meat spreads may be eaten.
- Do not eat refrigerated smoked fish, unless it is contained in a cooked dish, such as a casserole. Refrigerated smoked seafood, such as salmon, trout, whitefish, cod, tuna or mackerel, is most often labeled as "nova-style," "lox," "kippered," "smoked," or "jerky." The fish is found in the refrigerator section or sold at deli counters of grocery stores and delicatessens. Canned or shelf-stable smoked fish may be eaten.
- Do not eat raw or lightly cooked sprouts of any kind [including alfalfa, clover, radish, and mung bean sprouts].
- Cook sprouts thoroughly to reduce your risk for getting sick. Thorough cooking kills the harmful bacteria.
- When you're eating out, ask that raw sprouts not be added to your food. If you buy a ready-made sandwich, salad, or Asian food, check to make sure it doesn't contain raw sprouts.
- Refrigerate leftovers within 2 hours in shallow, covered containers and use within 3-4 days.
- Use a thermometer to make sure your refrigerator is 40°F or lower and your freezer is 0°F or lower.