“Is It Transient or a Harborage?” — A Thursday Night Listeria Call
Bill Brodegard
Thursday night, 5:45 PM. My phone buzzes. The FSQA manager from one of the sites I was leading. I picked it up immediately. “Hello?”
“Hey Bill. Just got a Listeria species positive. Zone 2.” I walk over to my computer and start to pull up the plant information. “OK,” I say, “What is that ID Number?”
Through the phone, I can hear my manager’s fingers clicking on her keyboard. She gives me the number, and I pull up the photo on my screen. Angle iron, no stacked washers, tack welds, or visible corrosion. The spot looked clean.
“Ok, Ok. Can you pull up the historical records for this area? Has there been any maintenance recently?"
More clicking on the keyboard. I am pulling up the same information. We had tested this area six times in the past year, all negative. There was a record of maintenance work last week, but I had pictures from the maintenance supervisor after the work was completed. They looked good. I checked the GMP and sanitation records from the night before just to make sure. All green.
I took a breath. “Alright,” I said. “Here’s what we are going to do.”
Beyond the Lab Result
A positive Listeria species result will make anyone’s pulse quicken. But what you do next — and how you do it — matters most.
The big question that Thursday was whether this was a transient positive (an incidental contamination) or a sign of a harborage (a place where Listeria is living, growing, and persisting in the plant).
A harborage can damage your business, cause product recalls, and put consumers at risk. But jumping to conclusions can be just as dangerous, leading to unnecessary shutdowns and panic.
In this case, we leaned on a mosaic of information to guide our decision:
Historical data — We had a full year of negative results from that exact area.
Sanitary design — The photo showed angle iron, not hollow tubing where water could stagnate and Listeria could grow.
Maintenance records — The equipment had been maintained properly, with no surprise corrosion or weld failures.
GMP practices — The sanitation team documented correct cleaning procedures.
Follow-up testing — We designed a vector swabbing plan, going back to the area for three weeks straight to look for recurrence. No further positives showed up.
Altogether, that gave us confidence: this was a transient positive, not a harborage.
The Power of a Mosaic
If there’s one takeaway from that Thursday night phone call, it’s this: lab data alone is never the whole story.
Lab results are one tile in the mosaic. You need to see the entire picture to truly assess risk. That means combining:
Trend analysis
Hygienic design knowledge
Maintenance records
Sanitation verification
On-site visuals (photos, videos)
Repeated, well-planned follow-up swabbing
All these tools let you prove — not just assume — that you have control of your environment. In this industry, confidence comes from convergence: multiple lines of evidence pointing to the same conclusion.
Next Time the Phone Rings
Next time you get that Thursday night call, remember:
Don’t panic.
Ask for the history.
Ask for the design details.
Ask for visual proof.
Plan your follow-up vector sampling.
By piecing those elements together, you can make the right call — protect the product, protect the brand, and protect the people who trust you.
Because at the end of the day, food safety is more than a test result. It’s the story behind the result that really matters.
If you’ve faced a similar Listeria puzzle, I’d love to hear how you approached it — let’s keep learning together.
About the Author
Bill Brodegard is a food safety and quality leader who’s spent over a decade guiding major food brands like Driscoll’s, Ajinomoto, and Schwan’s to build exceptional safety and quality programs. With a background in food science and a law degree focused on FDA/USDA regulations, Bill has combined hands-on experience with deep regulatory knowledge to drive meaningful change.
Now, Bill is channeling that experience into helping growing food businesses create outstanding food safety and quality systems. Because great food safety isn’t just for the big players—it’s for everyone who wants to build trust and deliver excellence.
