DIY – Listeria Testing

Should Small Producers do their own Environmental Testing? 

Environmental Listeria testing supplies

Recent regulations and high profile cases have made testing for listeria through the use of a certified laboratory somewhat discouraging. A positive test is no longer necessarily just between you and the lab. It is likely to be reported to the FDA. In some cases they must be notified. So what is a well-intentioned cheesemaker to do? For years we, farmstead and artisan cheesemakers and small dairy processors, have been told to NEVER test for pathogens on-site.  Too dangerous, too unpredictable, too little experience on the cheesemaker’s part.

After reading an article by respected dairy science educator Dr. Catherine Donnelly about environmental Listeria testing (see link below) ,it dawned on my –  if we do test for Listeria ourselves, AND we do find it, then we have way bigger concerns than not being certified lab technicians working in a certified facility. We are, in fact, growing it in our cheesemaking facilities. In addition, if good manufacturing practices are the norm, then you are unlikely to have any type of listeria in your plant, at least not for long, so testing will hopefully simply confirm negative results.  So why not learn the lowest risk, most accurate, least expensive way to test for this pathogen ourselves and become more aware, more proactive, and better producers of safe food? After all, if we can be trusted to produce safe food that goes directly into the mouths of people, why can we not be trusted to learn safe practices for self-testing?

Why are Listeria of such Concern?

Before I talk more about do-it-yourself tests, let’s talk about why Listeria is so high on the radar when it comes to food borne illness. (At the end of this article there is a link to the FDA’s booklet on food borne pathogens) The Listeria family are gram positive, facultative bacteria (meaning they can adapt to anaerobic conditions, even though their primary life process is aerobic). They are readily found in damp soil and moist environments. Fortunately, all but one species of Listeria are harmless to humans. L. monocytogenes, existing in several identifiable strains (serotypes,) is one of the most deadly causes of food borne illness. Even though the rate of listeriosis (a listeria infection) is low in comparison to other sicknesses caused by contaminated food, the death rate is the highest.

Once Listeria takes up residence in a food production facility, it is particularly difficult to get rid of – forming protective biofilms that are resistant to many common sanitizers, being salt tolerant (allowing their survival in brine and later growth on the surface of cheeses), able to grow at temperatures below freezing, and showing resistance to commonly used sanitizers through the activation of a pumping system (bacterial efflux pump) which can remove injurious substances from the bacterial cell. Recent cases of detection on non-food contact surfaces with recurrence later in product have increased the determination of enforcement officials to take action before product becomes tainted. Understandable.

Good manufacturing practices – part of a complete food risk reduction program – are designed to prevent the introduction of pathogens in the plant, remove them from the environment and surfaces when contact is unavoidable, and remove the likelihood of pathogen survival in products where their presence is a possibility – such as in raw milk cheese . Listeria, along with other pathogens, can be found in the dairy and in raw milk, so cheesemakers should assume that listeria will enter the plant and possibly the process – at some point. By assuming its presence and then taking steps to assure its destruction in product through process controls such as goal pH, final moisture content, proper aging, and of course, pasteurization. Post production contamination must also be addressed. Pathogens not regularly removed from the plant have the opportunity to form resistant biofilms and spread throughout the plant during cleaning practices which might offer them the opportunity to atomize and become airborne.

Listeria Testing Options

Insite swabs, easy to use, but high rate of false positives (brown)

Let’s go over the most common ways that L. monocytogenes (the one we need to worry about) and other Listeria family members can be detected. At all times while reading this, please remember that I am not a Ph. D, not a lab technician, not a medical doctor, and not advising you. I am sharing what we are doing and how it has affected our process, decision making, and knowledge.

  1. PCR assay – done only in specialized laboratories. Detects the presence of any portion of a bacterial cell (DNA). Does not differentiate between viable (cells that can grow, infect, and spread) or dead.
  2. Enrichment – A sample is enriched to encourage the growth of the bacteria being sought. Should only be done in a certified laboratory where proper safety techniques can be employed, including disposal. Can be L. monocytogenes specific. About 25-40.00 per test not including shipping.
  3. Non-enrichment plating – A sample is cultured on a plate without enrichment. Disposable plates, such as 3M Petrifilm, are commonly used. According to some (see links below) can be safely done when proper procedure is followed, by the producer. Qualitative and quantitative, non-specific for L. monocytogenes, but positive plates can be sent to a certified lab for confirmation.  About 7.50 per test (including peptone water and Quick-Swab)
  4. Self-contained swabs – a sample is cultured in a self-contained swab. Results are positive or negative (qualitative). High rate of false positives in cheese environments, see link at end of the article. About 5.00 per test.

How we did our Tests  – and what we found

Three duplicate tests, different results

I have included several links below for instructions, including a video, on properly using and interpreting the 3M environmental Listeria test plates. Since my goal is not to teach this technique, but to share our experience, I would encourage you to review these instructions and also seek the advice of a dairy science and laboratory specialist before you proceed.

In my first tests I used only the inexpensive and simple swabs by InSite. Boy did I want these to work, they were so easy to use!  I took six samples as directed and incubated for the initial time. After the first phase of incubation, two were presumptive positives. After the 2nd phase, four were presumptive positives. (see photo above) Needless to say, I was extremely disturbed, until I read the research (link below) indicating that false positives in a cheese environment are more common (up to 100% ) than not. The swab solution evidently also changes color in the presence of other bacteria common to cheese plants.

Next I sampled three surfaces with both the InSite swabs and 3M Quick swabs (to be plated after several other steps onto the 3M Petrifilm plates). I followed all instructions carefully, using aseptic techniques (thank you nursing degree for help learning those protocols) and incubated each in different incubators (as they both require slightly different temperatures and times). The samples, from a wall behind the vatunder the vat pedestal (where it is moist all the time and is sometimes not cleaned as well as the floor) , and the creamery’s main floor drain.  Of these, all three Petrifilm plates were negative and one InSite swab was presumptive positive.

Next I did a more extensive sampling in the creamery using only the Petrifilm environmental plates. I sampled a crack in the concrete floor where there is also a low spot that is wet longer than the rest of the floor, the base of the wall nearby, the wall behind and hidden by the sink, and a drain tube in a compressor in the aging room. All were negative.

Our plan is to continue with routine testing, picking new locations each time. By doing this I feel as though our cleaning and sanitizing program is more properly monitored. I also enjoy being “in the know” and on top of things!  You can bet if we do have any positive results that I will let you know if follow up tests show it eliminated – and what worked.  Our current regimen includes the routine use of gaseous ozone, and floor flooding with peracetic acid sanitizer. We also are extremely restrictive in regards to traffic in and out of the make room.

Small homemade incubator

Characteristics of a small, farmstead or artisan laboratory:

  1. Isolated from food production or dining areas, including sharing air handling or ventilation systems.
  2. Protected from untrained personnel, visitors, children, or trained but otherwise vulnerable personnel.
  3. Cleanable surfaces and equipment.
  4. A federal, state, and locally acceptable way to sterilize plates before disposal.
  5. Proper safety gear and apparel (not shared with food production facilities)
  6. Proper storage capability of new plates and swabs.
  7. Up to date instructions for the use of testing materials.
  8. Proper method for sterilization of plates after use, small autoclave can be purchased from Nelson Jameson (see below). Some sources suggest a soak in high concentration of chlorine or 190 proof alcohol.


Controlling Listeria – Environmental Plate Technology Hunts Down All Species, Catherine W. Donnelly, PhD

FDA’s “Bad Bugs” booklet:

3M YouTube video of how to use Listeria Petrifilm plates

3M Guide and Warnings for proper use of Listeria petrifilm plates–&fn=34870664611_PEL.pdf

Performance of two commercial rapid methods for sampling and detection of Listeria in small-scale cheese producing and salmon processing environments. (Abstract only)


Nelson Jameson for all the supplies needed


10 thoughts on “DIY – Listeria Testing

  1. I cannot tell you how much I appreciate your posts like this. I am a home milker & home cheesemaker. My general feeling is that all raw milk is safe and protects itself and all cheese is safe……assuming the dairy animals used are healthy & fed properly (not fed grain to the exclusion of forage, for example). Ted Beals has an excellent pathogen presentation at FTLCDF’s website that I reference often.

    I also am not a big fan of the standard commercial milk tests. I think their usefulness is very limited. I AM interested in confirming for myself and potential future customers (I would like to make & provide ghee & some cheeses via herdshare ) that my products are safe from pathogens. And I would like to be able to do it myself in a farm based lab.

    My question: why are non work surfaces like floor drains critical? Listeria exists and does breed, as you note, in cool, moist places like drains. But how would it get to the cheese? I have also read (if I told you the name of the cheesemaker you would know him but it escapes me at the moment…wait….Peter Dixon) that listeria does not survive on wood surfaces UNLESS they are sanitized. Why is this bacteria an issue today when it was not traditionally. Specifically, see this post:

    Clearly, they are not worried about listeria! So why is it an issue today? And how is it getting into cheese? Is it the lack of competing bacteria? Is our use of concrete & stainless an issue? Copper is a natural anti fungal, for example.

    • Thank you for your comments! My new book, due out in the spring, should be right up your alley. It is called (at this point!) The Small Dairy Advisor. Specifically designed to help the raw milk producer.

      You are absolutely correct, if a surface is occupied by many other bacteria, such as would be found on wooden cheese aging shelves, then listeria has little chance of moving in. In floor drains, where it is likely and also where other chemicals (even natural ones) limit the number of bacerterial species, it is possible for it to spread during cleaning,especially over enthusiastic water pressure cleaning, onto ceilings, into exhaust vents, etc. Then it drops down onto food contact surfaces and then gets onto the cheese or utensils.

      Remember, the consumer of today is becoming more vulnerable to things that most of us healthy, farm-living, raw milk drinking, veggie eating people can process without an issue. The population of immune suppressed individuals is growing.

      Peracetic acid is wonderful. It is bascically hydrogen peroxide and vinegar combined at a molecular level, so it breaks down to those and then on to oxygen and water. Very environmentally friendly. Ozone too breaks down quickly to oxygen. Juniper would be tough to use on a large scale and also not leave a residue that would impart flavor (unless I am making gin, I suppose! :-)). Even peracetic acid is quite expensive compared to the more conventional sanitizers.

      Peter is one of my mentors!!

  2. Another thought…..could you not use certain herbal extracts to kill and/or control listeria? For example, juniper, traditionally used in meat curing, is known to be active against L. monocytogenes?

  3. In-house testing – I liked the thought that environmental samples can be tested in-house and confirmed by an outside lab. I think the confirmation can be done accurately in-house as well.
    We at Micro-Identification Technologies, San Clemente,CA have developed a very very simple tool to confirm if the colonies are indeed Listeria spp. It takes only 3-4 min to confirm, needs only water as the only reagent and is way cheaper than the $7,50 confirmation cost that has been mentioned. Moreover this technology is AOAC approved as well. I think this technology can help you with implementing the food safety program.
    I would love to have an opportunity to talk about the technology with you and possibly demonstrate its effectiveness. I can be reached via email ( or phone 229-395-9837.

  4. Gianaclis and Dr Amit, I can’t thank you enough for your comments. I too want to make an in house lab. Like one commentor, I plan to make raw milk and cheeses, yogurts available through milk share program. In fact, I’m in the process of getting the shares set up as we speak. Also want to do lotion. Very important to have clean milk and follow proper sanitation. Gianaclis, where will your book be available? Dr Morey, I will email you, too.

  5. This is an oldie but I wanted to comment.
    I wonder if there could be a super cheap way for people to test food at home for listeria before consuming… it would be a lot more effective than pasteurization and I really am sick of the government telling people what they can and can’t eat, its doing a lousy job of keep truly unhealthy food additives out of commercial food as it is.

    • Awesome. Thanks Gianaclis
      Em, you are so right about the gov’t not keeping unhealthy additives out of commercial foods. Or GMOs too. I work in the bakery industry and unless the items are pure scratch, you’ll be having all that xxxxx stuff. Problem is that much of what we eat has all that stuff, even in a bakery. So sad.

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