Before you go looking for Listeria, you need to make sure that senior management across all functions is supportive of this task–especially when Listeria is found. Will someone (or some function such as QA) get blamed? Will enough resources be given? Will outward communications (e.g., to customers or FDA) be actively managed?

Or is the collective opinion “we do not need to know, because Listeria is surely not here”?

Most food companies make the correct strategic and ethical decision to test for Listeria. To that end, they typically test for the genus of Listeria species (L. spp.), and not specifically for Listeria monocytogenes (Lm). Thus, they do not end up with confirmation that Lm is present and therefore do not butt up against a regulatory zero-tolerance policy. But they get a strong indication that conditions are favorable to Listeria growth, so they can choose to assume that Lm is present and act accordingly. If corrective actions are taken to eliminate all Listeria, then one can justify that if Lm were present, it would be eradicated by these corrective actions.

Some companies test for a “marker organism” such as Listeria innocua. The thinking is the same as that above. This includes making the assumption that if Listeria innocua is found, then Lm may indeed also be present.

If your company has decided to look for Listeria, how do you go about doing so? The overall objective is to establish, validate, and execute an “environmental monitoring program” (EMP). Good starting points are the “Environmental Monitoring Handbook” published by 3M and Cornell University and a publication by the Institute of Food Technologists, “Design Elements of Listeria Environmental Monitoring Programs in Food Processing Facilities.” For seafood, the National Fisheries Institute has a publication on ready to eat seafood pathogen control.

Key to an effective EMP is sampling. Sampling is complicated to plan since the choices affect time and money. How many samples, where, and what are you sampling for (e.g., Lm, Listeria innocua, or L. spp.) ? These choices affect costs and how soon, or not, microbiological results are received. Random sampling never carries the day. Rather, sampling needs to be strategic—based on sound statistical principles, science, and  your own assessment of what might really be going on.

Building a useful EMP is hard work, and requires a lot of patience and a lot of data. Listeria does not sit still–plant conditions change all the time. Hence, a positive sample today does not guarantee that there will be a positive sample tomorrow (especially when microbiology results take days to obtain). A good EMP finds growth niches and transfer points, and helps determine the overall risk for nearby food becoming contaminated.

In addition, actual as-made equipment design is incredibly important. Equipment not made hygienically, or that has been changed over time (“to make it work better”), may end up being a root-source of Lm. A good review of proper hygienic design can be found in “Food Safety Equipment Design Principles” by the Foundation for Meat & Poultry Research & Education.

Here are some effective approaches used in the food industry:

1. Bring in an expert third-party consultant or company to do the work for you.
2. Utilize the services of a certified and vetted microbiology lab, and partner with them.
3. Purchase best-practice software which models an EMP, and directs sampling for you.
4. Review the FDA and USDA guidance documents for direction–and then speak with your local regulators. (Yes, this is not without some risk.)
5. Conduct multiple “swab-a-thons” in your plant, sampling anywhere and everywhere to gather microbiological data. As mentioned above, it can be notoriously difficult to establish patterns of harborage and movement–but having hundreds of data points can at least give you a snapshot for the microbiological cleanliness of the plant. Note that this is time-consuming and costly, but could pay dividends if this is one of your only options.
6. Study, study, study. Map and analyze traffic patterns in the plant, evaluate water use and water flows (e.g., to and from drains), sample equipment around product zones, look at data from pre-operational activities, look at past microbiological sampling data, and talk with the sanitation team about what they see.

Note that it is generally not recommended to sample actual product given the regulatory consequences if Lm is found!

Regardless of the tactics chosen, Lm is typically hard to find in a “clean” plant. And determining where it comes from (i.e., the root source) is even harder. Microbial testing is still the best way to do this. Techniques such as ATP testing, total plate count or Enterobacter testing, and PCR-assay Listeria test kits can help guide you. But those results are only as good as the level of technical thinking that is brought to the them. One must truly evaluate where Listeria growth niches might be, regardless of how easy or hard it is to access those locations.

See the Related Articles below to read the series.

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