Listeria is widely distributed in nature, found in soil, water, vegetation, and animal feces. And since it can be found in soil, it has the ability to contaminate vegetables and produce grown in that soil. This can also occur if contaminated manure is used as fertilizer.

Thus, it is quite easy for Listeria to get into a food plant on someone’s shoes, or equipment being brought in from outside, or ingredients coming from a supplier.

Given the ubiquity of Listeria, a good food safety team (and their management) always assumes that the organism is present somewhere in the plant. Even with regular microbiological sampling coming up negative (i.e., no Listeria found), that is not a reason to lower the level of vigilance. Ever.

The insidious nature of Listeria is such that it can hide, even in untoward conditions, and it can be moved around, up, down, and sideways. The same factors which allow Listeria to enter a plant allow it to be transported within that plant.

Let’s talk about “harborages” and “movement vectors.”

Harborages are all areas in a plant which have the necessary conditions for Listeria to grow. Biofilms (resistant coatings which protect the organism) can offer an additional level of security for the organism. Listeria may or may not grow, but it can certainly survive. These areas (e.g., underneath equipment, in drains) are also called “growth niches” in the scientific literature.

Movement vectors are those locations in which harborages intersect with people and equipment moving around the plant. These are the opportunities for Listeria to move. These points (e.g., a fork truck running through contaminated water) are also called “transfer points” in the scientific literature. An example:

• An employee accidentally drops Listeria-laden potatoes on the floor.
• The employee walks through those potatoes and across another production line to get a stainless steel scoop to clean up the potatoes.
• The employee walks back through the clean production line to scoop up the dirty potatoes, throw them in a trash container, and then put the scoop back.
• Another employee, not observing any of this, picks up that scoop later to sample ready to eat potato salad.

As you can infer, unbeknownst to the employees, the potatoes were harborages for Listeria, and the scoop (and footwear) were movement vectors likely resulting in Listeria getting into food.

Now think about all of the movement and travel of people and equipment that occurs in your own plant. You may even see the following:

• Fork trucks moving pallets from a warehouse to a product zone.
• Stainless steel carts wheeled from one line to another.
• Production operators carrying ingredients from one line to another.
• Maintenance tool boxes being set on the floor, and then carried to another location.
• Mixing implements being used and then hung for ready use later without being cleaned.
• Bowls containing work-in-progress being wheeled across production lines on their way to a cooler.
• Employees leaving their production line to get to a break area by walking across other production lines that use different allergens than theirs.

Best practices for Listeria control necessitate that harborages are eliminated and movement vectors are identified and controlled. Attacking Listeria in this way can reduce food safety risk dramatically. Note that the root source is almost assuredly a harborage site (a hiding spot for Listeria). So one of the best ways to find that harborage site is by identifying and mapping movement in the plant. Then follow the movement backwards and forwards, and up and down. You may find yourself surprised at where water can run or come from. And therein lie the “eureka” moments.

The objective is to identify the true root source of the Listeria, and eliminate it. This is very different than simply finding some Listeria and tossing sanitizer on it—this is very common, and people are lulled into believing that the Listeria has now been controlled. This is hardly ever the case if the root source has not been eliminated and if the movement vectors have not been identified and actively managed.

See the Related Articles below to read the series.

To sign up for a free subscription to Food Safety Tech’s weekly Newsletter, click here