Listeria Outbreak on Mushroom Farm killed 4 people | Good Management Practice


Today’s video will cover a health topic. The information is not intended or implied as a substitute for professional medical advice, diagnosis, or treatment. I encourage you to confirm any information obtained from it and review all information regarding any medical condition or treatment with a medical professional. All content is for general information and education purposes only.

Enoki mushrooms from Korea contaminated with Listeria have sickened 36 people in 17 states. Four death are reported.

The mushrooms were labeled as “Product of Korea.”

The alert comes a day after Montebello, California-based Sun Hong Foods Inc. recalled all enoki mushrooms imported from Korea after the Michigan Department of Agriculture and Rural Development found that a sample was positive for Listeria monocytogenes. According to the CDC, the illnesses started on dates ranging from Nov. 23, 2016 to Dec. 13, 2019, and 30 hospitalizations have been reported, with six pregnancy-associated cases, of which two resulted in fetal loss.

The CDC says Listeria can cause different symptoms, and invasive listeriosis usually starts one to four weeks after eating food contaminated with Listeria but can start as late as 70 days after exposure. Pregnant women typically experience fever and other flu-like symptoms. Still, infections during pregnancy can lead to miscarriage, stillbirth, premature delivery, or life-threatening infection of the newborn.

Symptoms in people who are not pregnant can include headache, stiff neck, confusion, loss of balance, and convulsions in addition to fever and muscle aches, the CDC said.

As of June 9, 2020, this outbreak appears to be over.

Video: Listeria Outbreak on Mushroom Farm killed 4 people | Good Management Practice


Listeria is a family of bacteria which consists of 10 species. One of them, Listeria monocytogenes, is responsible for the disease listeriosis. Listeria monocytogenes are widespread in the environment, so it is not surprising that it is occasionally found in food.

It can survive and grow over various environmental conditions, such as low pH and high salt concentration. It can survive on cold surfaces and multiply slowly at -4°C (24 °F), defeating one traditional food safety defense -refrigeration.
Refrigeration at 4°C (40 °F) stops the multiplication of many foodborne bacteria but does not kill most bacteria.

Listeria, therefore, can easily spread to other foods and surfaces. The foodborne pathogen has a very high mortality

Certain foods – including ready-to-eat refrigerated foods, unpasteurized milk, unpasteurized raw milk, and foods made with unpasteurized milk – often may be contaminated with Listeria monocytogenes, the third leading cause of
death from food poisoning.

The average incubation time is between 1 and 12 days, with a median of 2 days.

Fiver, muscle soreness, vomiting, diarrhea, sepsis, headache, stiff neck, confusion, loss of balance, and convulsions are symptoms of the infection. Infected people can excrete the pathogen in the stool for several months.

How to Prevent Listeria Infections at home


Wash your hands before, during, and after handling food. Wash utensils, cutting boards, and any surfaces that food touches after each use. Wash fruits and veggies—but not meat, poultry, or eggs.


Use separate cutting boards, plates, and utensils for raw (uncooked) produce and raw (uncooked) meat, poultry, seafood, and eggs. Keep meat, poultry, seafood, and eggs separate from all other foods while shopping and in
the refrigerator.


Only a food thermometer can make sure meat, poultry, fish, and casseroles are cooked to a safe internal temperature.
For example, internal temperatures should be 63°C (145°F) for whole meats and fish, 71°C (160°F) for ground meats, and 74°C (165°F) for all poultry. Eggs should be cooked until the yolk is firm.


Use appliance thermometers to ensure your refrigerator is at or below 4°C (40ºF), and your freezer is -18°C (0ºF) or below. Between 4°C (40ºF) and 63°C (140ºF) is the Danger Zone when bacteria can multiply rapidly. Generally, the more bacteria, the more likely someone will get sick. Most refrigerators have just a colder/warmer adjustment, so the only way to know is to put a thermometer inside.

How to avoid Listeria on your farm

In the EU, food is considered safe concerning Listeria monocytogenes if its numbers do not exceed 100 CFU/g throughout the shelf-life of the food.

Therefore, it is important to determine if food supports the growth of Listeria.

The objective of the first study was to investigate the growth of Listeria monocytogenes on both sliced and whole mushrooms of Agaricus bisporus as well as in mushroom casing and mushroom substrate.

The study found that the incubation of the casing layer at 20°C (68°F) for 50 hours increased the amount of Listeria monocytogenes to around 4 log CFU/g. At the end of the incubation period after 20 days, the number dropped to 2 log CFU/g.

Besides, the author found, after storing for 150+ hrs at 15°C (59°F), 7.3 log CFU/g on whole mushrooms and 9.5 log CFU/g on sliced mushrooms.


In the second research paper, the authors investigated the efficacy of current hygiene practices on the minimization of Listeria monocytogenes.

They compared the practice of two companies by analyzing the substrate and the floor of the fruiting rooms at various stages during the treatment process.

The postharvest hygiene procedure of the two companies is shown in the following figure (Figure 2).

The main differences between the two companies are that they use different chemical solutions.

Company 1 used a 1% sodium hypochlorite while company 2 a 2% Omnicide solution.

And second, how often both companies applied their solution.

While company 1 applied the solution after each steam cookout, company 2 only after the first round of steam cookout.

To measure the influence of these two treatments, the authors took swab samples at different stages during the whole treatment and analyzed them.

The first set of samples were taken before the treatment. For company 1, all samples of the floor samples were positive.
In contrast, for the second company, only 75 % of the samples were found positive.

When it comes to the substrate, for company 1, 25 % were found positive, for the second company, 42 %.

After the first steam cookout, the values changed as follows.

For company 1, the positive floor samples dropped down to 56 %. Whereas for company 2, 67 % of the floor samples
were found positive. For both companies, all substrate samples were negative.

The final samples were taken after the second round of treatment. The authors found 13 % positive samples from
the floor for the first company and 19 % for the second company.

The authors then investigated the reason for the positive floor samples and the difference in both companies. They found that for the first company, that the floor temperature during the first steam cookout reached only 45°C (113°F) while for the second company 55°C (131°F).

The substrate temperature reached 70°C (158°F) for 10 hours for company 1 and reached almost 70°C (158°F) for 10 hours for the company 2.

Both values indicate that in order to kill Listeria monocytogenes, the temperature has to be greater than 60°C (140°F).

To further investigate, the authors analyzed the heat tolerance of various Listeria monocytogenes strains. They found that after a cookout at 50°C (122°F), the mean D-value for this temperature was between 144 and 550 minutes.

The D-value is the time required, at a given condition or set of conditions, to achieve a log reduction, that is, to kill 90% (or 1 log) of relevant microorganisms. Meaning, after the treatment, 10% of the microorganisms are still present!

In our case, the D-value is written, for example, as D50 = 144.

If the same treatment were repeated, 90% of the remaining microorganisms would be killed, and the number would drop down to 1% remaining.

Increasing the cookout temperature to 60°C (140°F) and 65°C (149°F) decreased the D60-value down to 1.2 minutes and the D65-value down to 0.7 minutes.

These values indicate that the higher the temperature, the shorter the time needed to reduce the microorganisms by 90%.

But mushroom farmers should pay “particular attention […] in the construction of new mushroom growing houses to the insulation of the floors, to facilitate achieving higher floor temperatures during a cookout and ensuring
that microbial inactivation is achievable.

Alternatively, a steaming operation that targets floors specifically could be considered.

If you want to learn more about avoiding contamination, the following video is for you. In it, I talk about the six vectors of contamination. Talk to you there.


📝Leong, D.; Alvarez-Ordóez, A.; Guillas, F.; Jordan, K. Determination of Listeria monocytogenes Growth during Mushroom Production and Distribution. Foods 2013, 2, 544-553.,

📝Pennone V, Dygico KL , Coffey A, et al. Effectiveness of current hygiene practices on minimization of Listeria monocytogenes in different mushroom production‐related environments. Food Sci Nutr. 2020;8:3456–3468. 10.1002/fsn3.1629,