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Control of Listeria in Produce

“Control Strategies for Listeria monocytogenes in Produce Processing Facilities and Packing Houses”

Funded under the USDA Specialty Crop Research Initiative, contract number: C00135GG.

Based on the results of the research activities conducted under this project, a two-part webinar series was hosted by the Food Safety Lab at Cornell University and the Strawn lab at Virginia Tech. The goal of these webinars was to demonstrate a training program for Listeria control and environmental monitoring in produce facilities and packing houses. Links to the slide sets and recordings of each webinar are provided, below. 

Webinar 1: “FSMA and Listeria 101”

View Webinar 1 Recording | PDF of Webinar 1 Slides

Webinar 2: “Listeria Environmental Monitoring Programs”

View Webinar 2 Recording | PDF of Webinar 2 Slides


Relevant Resources


Frequently Asked Questions

Jump to a Question:

Q: What is the definition of Listeria contamination in a ready-to-eat food?

A: Presence (in any amount) of Listeria monocytogenes in a ready to eat food would mean the food would be considered as adulterated. In the US, there is no established acceptable level of L. monocytogenes in RTE foods, such as produce.

Q: Have antimicrobial surface coatings proven to be effective in inhibiting L. monocytogenes growth or spore germination?

A: There has been a surge recently in research relating to antimicrobial coatings for fresh produce. Although some research demonstrates the potential for coatings to minimize the growth of Listeria on produce during storage, it is important to be in the mindset of preventing Listeria contamination in the first place. Antimicrobial coatings can sometimes be used as a ‘crutch’ or a ‘band-aide’ that avoids the true issue of Listeria contamination during the packing stage. Although it may be worth investigating these coatings as an additional hurdle, it should not be used as the primary method of Listeria control.

Listeria does not form spores.

Q: Can Listeria grow below 41°F? Or below pH 4?

A: Listeria’s optimal growth range is 86 – 98°F (30 - 37°C), it can also grow, slowly, at refrigeration temperatures (as low as 32°F); Listeria thus grows at 41°F. It can also survive freezer temperatures (for example, in ice cream). The challenge with Listeria is few other organisms can grow at these low temperatures, and therefore Listeria has a reduced likelihood of being out competed by other microorganisms at these lower temperatures. Imagine, for example, a cantaloupe has Listeria on its rind, and someone cuts it into pieces and stores the cut melon in the refrigerator, which may or may not be properly cold. Then they eat the fruit throughout the week. By the end of the week, the Listeria has had plenty of time and nutrients to grow and cause potential illness.

Listeria does not grow below pH 4.4. However, there is the potential that L. monocytogenes could survive at pH below 4.4.

Food Standards Guide to L. monocytogenes

Q: Referring to the brush bed contamination in the cantaloupe case: How can we hold the brush roller distributors accountable? I have asked them how to clean their brushes and they never have any answer.

A: Unfortunately brush beds are identified as a common harborage site in many produce facilities and packing houses, as it is difficult to clean and sanitize them. While we agree there are few feasible ways to effectively clean and sanitize these brush beds, some strategies to reduce the likelihood Listeria persistence in brush beds include, removing as much organic matter (e.g., leaves) from the produce prior to reaching the brush bed, increased frequency of cleaning and sanitation to remove wax and organic material, and purchasing 2 sets of brush beds to alternate between to allow them time to dry out. You could consult a lawyer to see whether you could hold suppliers of food equipment that is uncleanable liable for problems associated with this.  It also is important to support industry groups in their efforts to work with equipment suppliers to make sure they supply equipment with appropriate hygienic design.

Q: Can you talk about the FDA EMP draft guidance from 2017 that outlines recommendations for frequency, zones, and expectations?

A: The 2017 draft guidance provides a more detailed explanation of what is required in an environmental monitoring program as stated in 21 C.F.R. § 117.165. The draft guidance includes a table with questions to help determine if the food product in question is considered high risk, a description of the classical 4-zone concept, and procedures and examples of corrective actions, among other sections. While the document does provide some specific details on what is to be done following a positive result, many parts remain vague as environmental monitoring programs should be tailored to what makes most sense for each facility and food product.

Control of Listeria monocytogenes in Ready-To-Eat Foods: Guidance for Industry Draft Guidance

Q: What risk does Listeria pose in dry areas?

A: Listeria prefers wet environments; however, this is not a hard and fast rule. There have been cases where Listeria has been isolated from dry environments. When developing an environmental monitoring plan it is best to focus on swabbing wet areas because you are more likely to find Listeria. With that being said, adding a few dry areas, particularly those with large amounts of organic matter, is generally a good idea. Additionally, dry area swabbing should be performed if routine swabbing indicates the possibility of a niche in a dry area. It's also good to swab in dry areas if that is where the finished product is located, to ensure that your control programs are keeping Listeria out of this area.

If you have a dry plant, you should also consider swabbing for Salmonella.

Q: Is there a graphic available of the physically molded brushes mentioned?Brush Bristle Comparison

A: The blue brush bristles are molded to the base, where as the yellow brush bristles are stapled to the bottom of the well. We are not aware of any brush bed rollers that are designed in this fashion.

Q: How can we reduce the hazards of transport of Listeria from farms to the packing house?

A: Remove as much debris and culls as you can before it makes it to the packinghouse (e.g., remove leaves, knock off dirt).

Have designated forklifts that unload the trucks and bring the produce to the packinghouse, and forklifts that bring the product into the bin dumper area. This is especially important on rainy days when mud can easily be spread around. If designated forklifts are not an option, consider having a quat powder or other sanitizer for the forklifts to regularly drive through when transitioning between different areas of the packinghouse.

Q: What is the difference between outbreak and incidence?

A: The WHO defines an outbreak as the occurrence of cases of disease in excess of what would normally be expected in a defined community, geographical area or season. The CDC defines incidence as the occurrence of new cases of disease or injury in a population over a specified period of time. In the context of our webinar, the incidence is individual cases, whereas outbreaks are cases tied to one specific food exposure. Even though we have approximately the same number of cases of Listeriosis over the last few years, we are detecting more outbreaks, because we are better at identifying smaller outbreaks (e.g., 2 cases) and tying these outbreaks back to a food source. This is because of advances in technology, such as whole genome sequencing.

Simply speaking, incidence represent all listeriosis cases that occur in the US in a given year, while outbreaks represent instances of two or more cases that are definitively linked to a given source (e.g., food X produced by company Y).

CDC: Lesson 3: Measures of Risk

Q: Is it good to not find Listeria during environmental monitoring?

A: Although it is good to not have Listeria in your operation, absence of Listeria positive testing results could also be an indication of an inadequate monitoring program. Repeatedly having negative samples may mean you need to identify new sampling sites, re-train on sampling techniques, or try a new sponge sampling tool. If you do not look in the correct places, are not rigorous enough in your sample collection, or are using a q-tip swabs instead of a sponge, you may not be detecting the Listeria that is present in your operation. If this still does not result in any positives, then it may be good to contact an outside expert for additional support or to conduct an external swab-a-thon.

Q: Does Listeria enter a VBNC state?

A: This question represents an active area of research with no definitive answer at this point. However, there is clear evidence that some Listeria found in processing plants may be difficult to culture and hence may not always be detected.  Use of validated methods that have shown sensitivity equivalent to culture methods is essential to minimize the risk of false negative results. Validation data should always be reviewed by an expert, as standard procedures used to validate methods for environmental swabs may overestimate the sensitivity of methods (particularly molecular methods with short enrichment times). This is because inoculation procedures for environmental samples (e.g., metal chips) typically lead to the presence of viable Listeria along with large numbers of non-viable cells whose DNA is still detectable.

Q: We have clients who want us to test for Listeria spp. instead of Listeria monocytogenes in apples and pear grading facilities. What is your opinion on this?

A: Testing for Listeria species is generally more conservative than testing for Listeria monocytogenes, as Listeria monocytogenes is included in Listeria species testing. However, if you do not have adequate resources to respond to every Listeria species positive, then it is helpful to be able to prioritize, which occurs when you test for only Listeria monocytogenes.

It is best to discuss with your clients that a strong environmental monitoring program with effective corrective actions will be more informative of potential contamination issues than final product testing for Listeria spp.

As a rule of thumb, environmental samples are typically tested for Listeria spp.  Finished product samples should be tested for L. monocytogenes (and not Listeria spp.); if a finished product sample is tested for Listeria spp. and tests positive for Listeria spp., then one must confirm whether the organism is L. monocytogenes or a non-pathogenic Listeria spp.

Q: Is it mandatory for farms or facilities to report the detection of pathogens to anyone? If not, do they have any incentive to do so? From the panelists' perspective, is this important data or is it best to keep that internal for the business to handle?

A: If pathogens are detected in the operation (i.e., in the environment) then there is no need to report this to a government agency. However, any pathogen testing, whether positive or negative, should be documented and kept in your records, along with the appropriate corrective actions. However, if a pathogen is found on a food contact surface, then the food produced on the associated equipment would be considered adulterated and needs to be recalled. If finished product tests positive, then the product also will need to be recalled (if it has been shipped) or needs to undergo a market withdrawal and cannot be shipped (if still under the control of the processor). Finished product positive samples need to be reported to the Reportable Food Registry (RFR).

FDA: Reportable Food Registry for Industry

Q: Do you recommend using a rapid test?

A: Rapid tests can be great because they are, of course, rapid. They can provide you with results days before a traditional test could. However, before using a rapid test, you should investigate and validate the test to ensure it minimizes false negatives (and false positives) when compared to traditional testing. Generally, any test that is used should have been validated by a certification body (e.g., AOAC). We would recommend use of validated rapid methods, particularly those that are also used by the FDA.

There are several types of rapid tests. In general, the faster the test, the greater the risk for false negative results. Additionally, we recommend you avoid rapid tests that use q-tip sized swabs, as the surface area is insufficient for effective environmental testing. If you decide to use a rapid test, it may be good to periodically send out samples for full culture-based testing.

Q: Do you have any guidance for swab-a-thon protocols (e.g., to set up PEM programs or deal with investigations)?

A: It can be helpful to bring in a 3rd party group to conduct a mock swab-a-thon to validate that your routine environmental monitoring program is effective at finding Listeria in your facility. Often experts who are not familiar with your plant are able to notice things you have become blind to, which will allow you to fix these potential problems before a regulatory agency performs a swab-a-thon in your facility.

Make sure swab-a-thon protocols allow for samples to be taken from beyond the original site lists. It’s good to have extra swabs, designated for use at the discretion of those who are picking out sampling sites.

Q: Thoughts on sanitizing prior to swabbing. I recognize a lot of manufacturers are washing, rinsing, sanitizing then swabbing.

A: You should collect sponge samples 3-4 hours into production to get the best idea of Listeria prevalence in your operation. This is because Listeria may be growing deeper in the equipment and does not get shaken out until machines are running and water is flowing.

You may want to swab after sanitation when you do follow up testing after an initial positive; here collecting samples after sanitation may help you to determine if the Listeria is surviving in the niche through cleaning and sanitation. In this case, you may want to test after sanitation and after the equipment has been running for a few minutes, but without product.

Q: Do you have any suggestions on how to share environmental monitoring results with your associates and management (e.g., percent positives, mapping, etc.)?

A: One of the best ways we have seen for sharing EM results with other relevant parties is via maps that are color coded to show positives from the same dates. This allows people to visually observe trending patterns over time and can often help spur discussions among colleagues about potential circumstances that led to a specific trending pattern. It can also be helpful to show negatives.

It can be good to show percent positives, although we recommend you include the numerator and denominator, rather than a lone percentage, to help put the numbers in context. Also consider breaking it down by zone, season, hygiene level, area of the operation, or other variables of interest.

Often pictures of positive sites can be useful for communication, particularly if they show what features of a site (for example, certain unhygienic design features) may be responsible for a site testing positive.

Q: My risk assessment and HACCP plan doesn't demonstrate a need for environmental monitoring (low-risk, specifically speaking to dry onion packing house). Do I still have to implement an EMP? We are a repacking facility, no "processing" or cutting of product. Would you want to consider EMP if they are conducting wet sanitation activities?

A: In general, for RTE products exposed to the environment after the kill step, an EMP is required. For low risk products, testing can be less frequent than for high risk products. For a dry onion packing house, we would still recommend some environmental testing for Listeria spp.

Q: Say you're relatively small grower/packer. You don't have a lot of resources but would like to start doing some EM. Would you recommend one sampling session with lots of samples, or few samples repeated over the season?

A: Both strategies can be helpful, but taking a few samples repeated over the season can help you to identify trends in where and when you are getting Listeria positives. You can then use these results to better understand problem areas in your plant. Also, you can start to tease apart what changed between positive and negative results, or vice versa, at a single site to guide the development of corrective actions.

Q: If you are just a cold storage, how often should you swab?

A: In general, cold storage facilities (assuming product remains in package throughout storage) are considered low-risk, and swabbing is not required under FSMA. However, environmental conditions in cold storage facilities are often ideal for the growth and survival of Listeria monocytogenes. As such, it is generally good practice to address potential situations that increase the likelihood of Listeria persistence in your facility, including drying up condensation from cooling units. It can often be a better use of resources to address such problems rather than swabbing. However, if there is exposed RTE product in a cold storage facility, we would recommend a PEM program that targets Listeria.

Q: Is swabbing to verify cleaning or to verify sanitation more important?

A: Verification of both cleaning and sanitation is essential, particularly since sanitation is not effective unless a surface has first been effectively cleaned (as they say “you can’t sanitize dirt”). Verification of cleaning uses ATP testing, while verification of sanitation typically involves testing for total bacterial counts [total plate count (TPC) or aerobic plate count (APC)]; these tests are discussed in more detail in the 3M handbook:

3M Handbook

Listeria testing would typically verify the overall Listeria control program, not specifically cleaning or sanitation.

Q: From a buyer’s perspective, how do you suggest we best support packing facilities that are using water in their process in implementing and improving EMPs and cleaning and sanitation?

A: It is important for buyers to encourage packing facilities to have a written sanitation program that includes the cleaning and sanitation of relevant equipment, such as the bin dumber and brush bed areas. Water should be used sparingly in packing areas and on equipment that is normally dry. However, it is also important for buyer’s to be realistic, and not expect packers to spend millions overnight to improve their food safety programs. Continue to encourage, or in some cases require, and prioritize the various cleaning and sanitation efforts and EMPs. Importantly, certain buyer requirements may have the unintended consequences of discouraging implementation of appropriate EMPs (for example, requiring suppliers to report all EMP Listeria positives to a buyer may encourage them to “sample to not find”). It’s important for buyers to try to consider potential unintended consequences like this.

Q: What are acceptable and unacceptable CFUs for Listeria spp. in environmental samples?

A: Environmental samples are generally tested to determine whether samples are positive or negative, with no determination of the CFU present. However, any amount of Listeria should be treated as “positive” and responded to using appropriate corrective actions.