Listeria positive environmental swab: the first 24 hours

The laboratory phones at two on a Thursday afternoon. Listeria spp. detected on the assembly belt you swabbed on Monday. The line is running, three days of product have gone out, and the first instinct on the floor will be to grab the foam gun. That instinct is the first mistake.

This post gives you the first 24 hours. The full method, and a positive drill to rehearse it before the phone rings, is in Practical Pathogen Environmental Monitoring (EMP) and Surface Swabbing from ASC Food Safety Training: 9 hours, online, self paced, R1 750.

What should you do first when an environmental swab tests positive for Listeria?

When an environmental swab tests positive for Listeria, write down exactly what the laboratory reported, confirm the sample site, date and zone against your swab plan, and start your positive result procedure. Then, before any intensified cleaning, take vectoring swabs around the positive site. Decide on product hold and whether the line keeps running by your written procedure, not the mood in the room.

Ask the laboratory four things: is the result presumptive or confirmed, has the species been identified, has the isolate been kept, and how long will identification take. Log the time.

The belt has had three routine cleans since Monday, and they did not remove the source. A panicked deep clean at double strength knocks counts down just far enough to give false comfort for a fortnight. Swab first. Vectoring takes an hour or two, so it is no excuse to delay the clean, or to keep running exposed product over a positive Zone 1 surface. Module 5 of the Listeria environmental monitoring course covers vectoring and acting on a positive.

What does a Listeria spp. positive actually mean?

Listeria spp. is a genus level result. It tells you that a member of the genus was recovered, not that Listeria monocytogenes, the species that causes listeriosis, is present. Sites test for the genus because it shares the same niches, so a positive means the site could support the pathogen.

So it is an early warning. Until the laboratory identifies the species, a careful site treats a Listeria spp. result on a product contact surface as if it could be L. monocytogenes. Identification adds days, so the hold decision cannot wait for it.

Zone matters as much as species: an assembly belt carrying exposed food is Zone 1, and a corridor drain calls for a different response.

If zoning is new to you, start with our guide to building an environmental monitoring programme for Listeria.

Does product have to go on hold after a positive environmental swab?

A Zone 1 positive puts product in question because food touched the surface. The affected product is everything that ran over that surface since the last point you can show it was under control. Your site procedure, your customers’ requirements and the law decide what is held, tested, released or withdrawn.

If the last clean result on that belt was four weeks ago, “just Monday’s production” is hard to defend. Consider shelf life, whether the product supports growth of L. monocytogenes, whether the consumer will cook it, and who eats it.

Finished product testing cannot prove a batch is free of a pathogen that is unevenly spread at low levels, so negative results support a decision without making it for you. Some customers require notification within a set time. The 2017 to 2018 South African listeriosis outbreak, with more than a thousand confirmed cases, is why nobody here should take that call lightly.

A hold decision made badly costs product, customers or both. Module 5 of the environmental monitoring and surface swabbing course works through hold and test and the return to routine frequency.

Start the EMP and Surface Swabbing course, R1 750

What are vectoring swabs, and why take them before you clean?

Vectoring swabs are extra samples taken in a widening pattern around a positive site: above, below, upstream, downstream and along the routes people, water and equipment travel. They are taken before intensified cleaning because their job is to find where the organism is coming from, and cleaning removes that trail.

For the belt, think in three dimensions: the underside and return path, the rollers, the scraper, the frame, the cable tray above, the drain beneath, the infeed before it and the packing station after it. Number each sponge on a sketch.

A small tip swab is the wrong tool for a large surface. Use a pre moistened sponge with neutralising broth, and get the samples to an accredited laboratory cold. Technique is in our surface swabbing guide to sponges, swabs and neutralising broth.

Then clean, re-sample and count the clean days

After vectoring, strip further than the nightly clean does, swab what you expose, then clean, disinfect and re-sample, both after cleaning and a few hours into production, when vibration and moisture bring organisms out of hiding.

One negative proves little. Many programmes require a run of consecutive clean results at the positive site and its vector sites before returning to routine frequency. The course calls this “the three clean days”. Your procedure or customer may demand more, and any positive during the run resets the count.

How do you tell a transient Listeria from a resident strain?

A transient contaminant arrives on a wheel, a crate or a boot, is found once and is removed by normal cleaning. A resident strain has settled in a harbourage site and keeps coming back at the same or nearby sites. Repeat positives after intensified cleaning point to a resident, and subtyping of isolates can confirm it.

A resident strain does not live on the surface you swabbed. It lives inside something, and the surface is where it shows up.

Finding it is called seek and destroy. Dismantle the equipment, swab as you go and look for places that stay wet and never see a brush: hollow rollers, bearing housings, cracked belt edges, worn gaskets, rough welds. Then remove the niche by repairing, redesigning, heat treating or replacing the part.

Close the event like any other nonconformity, with root cause and CAPA. Actions usually land in hygienic design, the cleaning method, traffic control or the sampling plan.

What should you tell the production manager?

Tell the production manager four things: what was found and where, what is not yet known, what is happening to product, and what you need from production today. Put it in writing within the hour. Do not speculate about the source.

“Listeria spp. was detected on the line 2 assembly belt, a product contact surface, sampled Monday. We do not yet know if it is L. monocytogenes. Product from the window set in our procedure is on hold. I need the line from 15:00 for swabbing, a fitter from 18:00 to strip the belt, and nobody to pressure wash anything until I say so.” High pressure hoses spread contamination as aerosol.

What does the Practical Pathogen Environmental Monitoring course cover, and what does it cost?

Practical Pathogen Environmental Monitoring (EMP) and Surface Swabbing, from ASC Food Safety Training, is an advanced, nine hour online course of six modules and 25 lessons. It costs R1 750 once, with lifetime access. Module 5 covers acting on a positive, and the module 6 workshop includes a positive drill in the EMP Programme Builder.

Enrol in Practical Pathogen Environmental Monitoring and Surface Swabbing, R1 750

Questions people ask before they enrol

How long does the environmental monitoring course take?

The course is nine hours, split into six modules and 25 lessons with 15 narrated videos and six games. It is self paced with lifetime access, so you can return to module 5 when your own laboratory phones.

What does the course cost, and is there anything free to try first?

The course costs R1 750 once, with lifetime access and a certificate on passing. A free EMP builder is available from the course page, so you can try the programme building tool before you enrol.

Does it suit dry and ambient plants, or only chilled ready to eat sites?

It covers both. Module 2 deals with wet and chilled pathogens and with dry and ambient pathogens and toxin formers, and module 1 explains how Listeria and Salmonella live in a plant.

Can I buy the course from outside South Africa?

You can. The price is in rand, and the course page shows an indicative equivalent in US dollars, euros and pounds. Payment settles in rand at your bank’s exchange rate on the day.

The worst time to design your response to a positive is at two on a Thursday with the line running. Write the procedure now and rehearse it. One week of held product costs more than R1 750, and a recall costs far more. Enrol in the pathogen environmental monitoring (EMP) course and get the method and the drill. For the rest of our microbiology training, see the food microbiology training hub.

This post is general guidance, not advice on a specific incident. Product disposition is decided by your site’s procedure, your customers’ requirements and the law. The course is an independent training product and is not approved or endorsed by any standard owner or certification body. Mzansi Fresh Meals is fictional.