Listeria in Leafy Greens

Information about Listeria monocytogenes and what can we do about it.

Salad containing leafy greens! I’m sure this one is fine. Image from Pixabay

Currently there are four FDA investigations due to outbreaks linked to leafy greens. Two are Listeria monocytogenes in packaged salad¹, one is E. coli O157 H7 in packaged salad² and the final is E. coli O157 H7 in Romaine³. 

Four outbreaks! 

That is 41 people ill and three deaths.

Three deaths! 

All due to pathogenic contamination of leafy greens.

What is especially bad about these deaths, which were caused by the Listeria outbreaks, is that in the biggest, probably caused by the contamination in Dole packed salad, the first illness occurred in 2014. This was a one off, but since 2018 there have been 16 more cases.  In the second, probably caused by contamination in Fresh Express packed salads first had outbreaks in 2016!

A total of four of Dole’s facilities are involved in the most recent version of the salad recall. 

Listeria monocytogenes is a pathogen that is particularly risky during pregnancy as it may cause infections which lead to miscarriage, stillbirth, premature delivery, or life-threatening infection of the newborn. There is some fascinating science here⁴.

Unlike other pathogenic bacteria, Listeria survives and grows at cold temperatures, even down to 32 F. These are perfect conditions for processing and washing lettuce as low temperatures will stop the leaves from respiring to fast and the high water content means that lettuce is easily damaged if it is frozen.  

Listeria has been indicated in many outbreaks and is particularly concerning in cold stored food because of its ability to grow at refrigerator temperatures and to form and persist in biofilms.

Biofilms are complex communities of microorganisms which are connected by a extracellular matrix, made up of polysaccharides, proteins, lipids and DNA. These biofilms attach to surfaces, such as food contact surfaces, our teeth (plaque is a biofilm) and water pipes.  Bacteria in biofilms are less susceptible to antibiotics and sanitation agents.

Bacteria that cause foodborne illnesses form biofilms on food preparation and processing surfaces in the home and industrial settings and Listeria monocytogenes is one that exists in biofilms. 

Other outbreaks related to Listeria monocytogenes⁵ in the past included Blue Bell Ice Cream in 2015 and Tyson’s frozen, fully cooked chicken in 2021 and ready-to-eat produces were the cause of the world’s largest listeriosis outbreak in South Africa in 2018 in which over 1000 people were ill and there were over 200 reported deaths⁶.

I’ve always disliked prepacked salad. Mostly because it seems an unnecessary and unsafe extra step to chopping up lettuce yourself. To have the shelf life required to transport lettuce across the country and sit on a shelf for few days, means that it is washed in sanitized water and then packed in a bag or a box. If packed in a bag, it is often puffed up with nitrogen or air so that the salad is protected during transit. 

We eat salad because we are told that is is “good” for us and adds micronutrients to our diet. Perhaps we need to reconsider this and how we grow and package our leafy greens. Instead of having lettuce grow in a desert in large fields with CAFOS nearby, we need support for local and  regional food systems, reduce harm to the soil, support the people growing and harvesting our food, and make sure the food will be safe for the people eating them.

I’m concerned about the mass manufacturing of food, especially of food like leafy greens that are ready-to-eat and susceptible to pathogenic bacteria. I would prefer local sourcing for fresh produce wherever possible.

Listeria monocytogenes can be controlled and managed with a thorough cleaning and sanitation program. Environmental monitoring is an excellent way to find out if there is a build up of listeria biofilms on non-food contact surfaces. If you are interested in learning more about developing a sanitation and monitoring program to manage foodborne pathogens, book a call today.

1. https://www.fda.gov/food/outbreaks-foodborne-illness/outbreak-investigation-listeria-monocytogenes-dole-packaged-salad-december-2021 and https://www.fda.gov/food/outbreaks-foodborne-illness/outbreak-investigation-listeria-monocytogenes-fresh-express-packaged-salad-december-2021

2. https://www.fda.gov/food/outbreaks-foodborne-illness/outbreak-investigation-e-coli-o157h7-power-greens-packaged-salad-january-2022

3. https://www.fda.gov/food/outbreaks-foodborne-illness/investigations-foodborne-illness-outbreaks

4.

5. https://www.cdc.gov/listeria/outbreaks/index.html

6. https://www.afro.who.int/news/who-congratulates-south-africa-end-worlds-largest-listeriosis-outbreak