Listeria monocytogenes Contamination in Produce- an Increasing Cause for Concern!

When it comes to produce safety, the most feared pathogens are E. coli O157:H7 and Salmonella sp.. It makes perfect sense given that several of the outbreaks and recalls related to contaminated produce have been due to E. coli O157 or Salmonella sp.. But, if we think about it, these pathogens were of major concern mostly in meat and egg products, which are now alarmingly isolated from various food sources, such as produce. On the other hand, Listeria monocytogenes, a saprophyte and a pathogen usually isolated from dairy products, cold-cut meats, and smoked fish have also made their way into fresh produce, causing several costly food recalls and multiple foodborne disease outbreaks leading to multiple illnesses and deaths over the past few years.

There was a very recent recall in June-July 2019 due to possible Listeria monocytogenes contamination in packaged produce, such as butternut squash, cauliflower, zucchini, and a butternut squash based veggie bowl. The recalled products affected multiple product brands and retail distributors (3). Currently, in November 2019, several other recalls have been initiated in various produce types due to possible Listeria monocytogenes contamination in produce by numerous distributors and retailers. Affected products include veggie trays, broccoli, tomatoes, snap peas, veggie power blend, and stir fry blends to name a few (4). Besides these recalls, there have been few major outbreaks in the past years. Outbreaks due to Listeria monocytogenes in produce in April to May 2016 caused multiple illnesses including some deaths. In this outbreak, Listeria monocytogenes caused 9 illnesses in four states due to the consumption of contaminated frozen vegetables. All of the nine recorded cases were hospitalized, with three reported deaths. This outbreak involved an initial recall of 11 frozen vegetable products, which was expanded to 350 consumer products sold under 42 separate brands (1).

Similarly, there was another outbreak in July 2015 to January 2016 due to Listeria monocytogenes contamination in packaged salads. This outbreak was reported to cause 19 illnesses in nine different states leading to hospitalizations of all the cases, with one reported death. The outbreak strain of Listeria monocytogenes was traced back to a particular processing facility in Springfield, Ohio. Several brands and varieties of packaged salads were recalled during this outbreak (2). Another outbreak in 2014-2015 due to Listeria monocytogenes contamination in Caramel Apples was attributed to 35 reported illnesses in 12 different states. This outbreak caused 34 hospitalizations leading to 7 deaths.

It is important to note that hospitalization and death rates are higher for patients with  Listeriosis. This is because of the greater mortality rate due to Listeria monocytogenes, which is as high as 30% (5). Also, the infection due to Listeria monocytogenes is mainly troubling to immunocompromised patients, i.e. elderly populations and children, including pregnant women. Listeriosis in pregnant women can cause miscarriages, stillbirths, and preterm labor. Produce are considered healthy foods given their nutritious values, that is why all people, including sick and vulnerable populations are encouraged to eat more fresh fruits and vegetables. However, it would be alarming to have situations where Listeria monocytogenes was routinely or more abundantly isolated from such food sources. It is important to understand the prevalence of Listeria monocytogenes contamination in produce and to implement mitigation strategies to reduce their related food safety risks.

Listeria monocytogenes is widely distributed in nature, such as in soil, water, and manure sources. Prevalence of these organisms are often found in niches in the processing facilities where it might be damp and cold. Usually, areas that are under minimal usage and seldom cleaned can also harbor these pathogens. A study review found that environmental, soil, and water samples collected from various sources showed 5.5% to 33% prevalence for Listeria monocytogenes (6). Fortunately, the prevalence of Listeria monocytogenes in fresh produce itself is not as high as that observed in environmental samples. A survey conducted by Aparecida et al., in New Zealand showed that only 3 samples (collard greens, parsley, and spring onions) from 162 leafy green samples were positive for Listeria monocytogenes (7). Another study published in 2012 showed that 3.1% of the samples collected in Brazil were positive for Listeria monocytogenes contamination with some of the samples with counts at high as 2 log CFU/g (8). Here in the USA, a survey conducted by Luchansky et al., for Listeria monocytogenes prevalence on raw cut vegetables (1,689), low acid cut fruits (2,408), and sprouts (2,652) showed that 18, 9 and 3 samples, respectively, for each product type were positive for Listeria monocytogenes. The reported levels of Listeria monocytogenes ranged from 0.036 MPN to 3 logs CFU/g in produce samples (9).

Once present on the produce samples, Listeria monocytogenes can survive for long periods of time because this pathogen is adapted to such conditions. Listeria monocytogenes not only survives but can also grow at temperatures as low as 0°C. It is also known to endure low pH conditions and survive and grow at high salt concentrations. These particular attributes of Listeria monocytogenes helps it adapt to cold storage temperatures or acidic conditions, which also happens to be the suitable storage parameters for fresh produce. These situations provide a favorable condition for a potentially increased survival and growth of Listeria monocytogenes.

It is equally critical to understand the sources of contamination of Listeria monocytogenes in produce. Mostly, studies are focused on understanding the prevalence and persistence of Salmonella sp., and E. coli O157 in produce fields or retail establishments. However, prevalance of  Listeria monocytogenes in produce-related environments is also beginning to increase in recent years. Some similar patterns and trends as to those for the spread and contamination of Salmonella sp., and E. coli O157 from various sources to produce farms are also noted for Listeria monocytogenes. A research study published in 2017 by Pang et al., studied the prevalence of Listeria sp., including Listeria monocytogenes, in mixed produce and dairy farms over a period of 14 months. They reported that wind speed and precipitation had a significant impact on the probability of isolation of Listeria sp. Their findings suggested that run-off during precipitation and wind-driven dust from one farm to the other may be reasons for contamination of produce on the produce farms (10). Besides, these indirect sources, contamination can occur on produce fields through the use of contaminated manure, irrigation water, and wild animals (11). Even in the processing facilities, there are several harborage sites and Listeria monocytogenes can survive for prolonged periods of time in these niches. Studies shows that Listeria monocytogenes can be persistent in such environments for years and are difficult to eradicate. However, many of these studies are conducted in meat and poultry facilities or delicatessen establishments, and data from produce processing facilities are lacking! Nevertheless, it is evident that such harborage sites can lead to contamination of produce in the processing facilities (12).

For proper mitigation of Listeria monocytogenes contamination in produce, a hollistic approach is needed that involves pre-harvest and post-harvest ramifications. Just as important is the responsibility of the consumers to prevent cross-contamination at homes and discard recalled and foodborne disease outbreak associated products with longer shelf life, mainly frozen vegetables/fruits, whenever such information is available. Information on product recalls and foodborne disease outbreaks are publicly available on FDA (Food and Drug Administration- and CDC (Centers for Disease Control and Prevention- websites.


  1. Multistate Outbreak of Listeriosis Linked to Frozen Vegetables (final Update). Posted July 15, 2016 1:45 PM ET. Retrieved on November 12, 2019.
  2. Multistate Outbreak of Listeriosis Linked to Packaged Salads Produced at Springfield, Ohio Dole Processing Facility (Final Update). Posted March 31, 2016 9:00 AM ET. Retrieved on November 12, 2019.
  3. Growers Express Issues Voluntary Recall of Multiple Fresh Vegetable Products Due to Potential Contamination of Listeria monocytogenes. FDA Public Date July 01, 2019. Retrieved on November 12, 2019.
  4. FDA Recalls, Market Withdrawals, & Safety Alerts. Current as Of November 12, 2019. Retrieved on November 12, 2019.
  5. Scallan, E.; Hoekstra, R.M.; Angulo, F.J.; Tauxe, R.V.; Widdowson, M.A.; Roy, S.L.; Jones, J.L.; Griffin, P.M. Foodborne illness acquired in the United States—Major pathogens. Emerg. Infect. Dis. 2011, 17, 7–15
  6. Zhu, Qi, Ravi Gooneratne, and Malik Hussain. Listeria monocytogenes in fresh produce: outbreaks, prevalence and contamination levels. Foods 3 (2017): 21.
  7. Aparecida, O.M.; Abeid Ribeiro, E.G.; Morato Bergamini, A.M.; Pereira De Martinis, E.C. Quantification of Listeria monocytogenes in minimally processed leafy vegetables using a combined method based on enrichment and 16S rRNA real-time PCR. Food Microbiol. 2010, 27, 19–23.
  8. Sant’Ana, A.S.; Igarashi, M.C.; Landgraf, M.; Destro, M.T.; Franco, B.D.G.M. Prevalence, populations and pheno- and genotypic characteristics of Listeria monocytogenes isolated from ready-to-eat vegetables marketed in São Paulo, Brazil. Int. J. Food Microbiol. 2012, 155, 1–9.
  9. Luchansky, John B., et al. Survey for Listeria monocytogenes in and on ready-to-eat foods from retail establishments in the United States (2010 through 2013): assessing potential changes of pathogen prevalence and levels in a decade. Journal of food protection 6 (2017): 903-921.
  10. Pang, Hao, et al. Identifying and modeling meteorological risk factors associated with pre-harvest contamination of Listeria species in a mixed produce and dairy farm. Food research international 102 (2017): 355-363.
  11. Smith, Alva, et al. Sources and survival of Listeria monocytogenes on fresh, leafy produce. Journal of applied microbiology4 (2018): 930-942.
  12. Buchanan, R.L., Gorris, L.G.M., Hayman, M.M., Jackson, T.C.and Whiting, R.C. (2017) A review of Listeria monocytogenes: an update on outbreaks, virulence, dose-response, ecology, and risk assessments. Food Control 75, 1–13.



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