Should Small Producers do their own Environmental Testing?
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
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.
- 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.
- 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.
- 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)
- 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
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 vat, under 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.
Characteristics of a small, farmstead or artisan laboratory:
- Isolated from food production or dining areas, including sharing air handling or ventilation systems.
- Protected from untrained personnel, visitors, children, or trained but otherwise vulnerable personnel.
- Cleanable surfaces and equipment.
- A federal, state, and locally acceptable way to sterilize plates before disposal.
- Proper safety gear and apparel (not shared with food production facilities)
- Proper storage capability of new plates and swabs.
- Up to date instructions for the use of testing materials.
- 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 http://www.foodquality.com/details/article/834047/Controlling_Listeria.html?tzcheck=1&tzcheck=1
FDA’s “Bad Bugs” booklet: http://www.fda.gov/downloads/Food/FoodborneIllnessContaminants/UCM297627.pdf
3M YouTube video of how to use Listeria Petrifilm plates: http://www.youtube.com/watch?v=f-x1PYFK19w
3M Guide and Warnings for proper use of Listeria petrifilm plates http://multimedia.3m.com/mws/mediawebserver?mwsId=SSSSSufSevTsZxtUOY_15YtZevUqevTSevTSevTSeSSSSSS–&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) http://www.ncbi.nlm.nih.gov/pubmed/22960690
Nelson Jameson for all the supplies needed: http://nelsonjameson.com/splash-page.html