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Listeriosis - Outbreaks in packaged salads

Ready-to-eat foods such as salad mix, cut up fruit and pre-packaged sandwiches or entrees are highly sought-after for their nutritional value, convenience and reasonable prices. The consumption of pre-prepared fresh food is on the rise worldwide and there are more kinds of ready-to-eat foods than ever before. All-in-all ready-to-eat food products are a big time saver and can even be less expensive than purchasing all the ingredients and preparing the meal yourself!

However, one should be mindful of disease-causing pathogens such as bacteria that may be lurking in freshly manufactured foods. News outlets and social media are helping to bring increasing worldwide awareness of foodborne illnesses. Essentially a foodborne illness, also called food poisoning, is an illness caused by food harboring unwanted pathogen, chemical or physical agents. A pathogen is a bacteria, virus, parasite or other infectious agent that causes disease:

  • 1. Foodborne illness infections occur when pathogens proliferate in the intestines of individuals who ate pathogen contaminated food.
  • 2. Foodborne illness intoxifications occur when ingested pathogens produce toxins in the intestines.
  • 3. Foodborne illness intoxications occur when an individual eats pre-existing toxins that had been produced by a pathogen in the food.

News outlets, social media and government agencies are helping to bring increasing worldwide awareness of foodborne illnesses. For example in January 2016, officials from the FDA (U.S. Food and Drug Administration) confirmed the bacterium Listeria monocytogenes in a packaged salad mix produced at the Dole Fresh Vegetable Inc. plant in Springfield, Ohio. As a result, Dole voluntarily recalled all the salad mixes produced in the Springfield, Ohio processing facility. The salad mix products were sold under multiple brands in the U.S. and Canada.

Listeria is a Gram-positive bacterium belonging to the phylum Firmicutes, the order Bacillales, the class Bacilli and the family Listeriaceae. Listeriosis is a serious pathogenic infection usually caused by eating food contaminated with Listeria monocytogenes. Listeriosis is also an important public health problem with the disease primarily affecting pregnant women, newborns, adults with weakened immune systems and older adults. Infections during pregnancy can potentially lead to miscarriage, stillbirth, premature birth, or infection in the newborn baby. However, foodborne illness is difficult to diagnose and most people with symptoms do not seek medical attention.

Underdiagnosis and reporting coupled with the rapid distribution of food on both national and global scales makes it difficult to detect a foodborne outbreak in time to limit its impact. The listeria outbreak had actually been going on for more than half a year before the bacterium was definitively linked to salad greens from the Dole facility. At least fifteen people across eight U.S. states were confirmed with listeriosis in the Dole outbreak. All fifteen required hospitalization and one died. Additionally, public officials in Canada reported that at least seven people across five provinces were infected with listeriosis linked to the Dole outbreak.

Risks from listeria and other foodborne illnesses can greatly be reduced by following recommendations for safe food preparation, consumption and storage. However, food-borne illnesses, outbreaks and recalls are a reality of the food supply life cycle.

Listeria, like many other foodborne pathogens, is naturally found in the environment and people are regularly exposed to it. For example, listeria bacteria are found in soil, decaying vegetation and water. Listeria can also be part of the normal bacterial fecal flora of humans and other mammals. Some studies suggest that up to 10% of the human population may be intestinal carriers of listeria.

It is important to keep in mind that most people do not become sick from exposure to listeria nor develop listeriosis. For those who do, it is usually mild and self-limiting. Mild listeriosis may consist of influenza type symptoms such as fever, muscle aches and diarrhea. In healthy people listeria activates the immune response including the production of TNF (tumor necrosis factor) and other cytokines. These cytokines, in turn, activate immune cells such as macrophages and monocytes to attack and kill the listeria bacterium. Life-threatening forms of listeriosis include septic shock, meningitis and encephalitis.

Researchers are continually seeking ways to detect as well as understand and study the basic mechanisms underlying the pathology of listeria infection. MyBioSource is a research reagents company developing leading-edge tools and technologies for studying listeria and other infectious agents. Products for studying listeria include real-time PCR and ELISA Kits, antibodies, and recombinant proteins:


  • Listeriosis - Outbreaks in packaged salads
  • MyBioSource
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