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ARS Home » Pacific West Area » Albany, California » Western Regional Research Center » Produce Safety and Microbiology Research » Research » Publications at this Location » Publication #379425

Research Project: Ecology and Detection of Human Pathogens in the Produce Production Continuum

Location: Produce Safety and Microbiology Research

Title: Growth and survival of attached Listeria on lettuce and stainless steel varies by strain and surface type

Author
item Gorski, Lisa
item Walker, Samarpita
item Romanolo, Kelly
item KATHARIOU, SOPHIA - North Carolina State University

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/29/2020
Publication Date: 1/7/2021
Citation: Gorski, L.A., Walker, S., Romanolo, K.F., Kathariou, S. 2021. Growth and survival of attached Listeria on lettuce and stainless steel varies by strain and surface type. Journal of Food Protection. 84(5):903-911. https://doi.org/10.4315/JFP-20-434.
DOI: https://doi.org/10.4315/JFP-20-434

Interpretive Summary: Listeria monocytogenes can cause severe foodborne illness in the immunocompromised, the elderly, and result in spontaneous abortion during pregnancy. Annually, L. monocytogenes is estimated to cause 1,600 invasive infections in the U.S. resulting in a hospitalization rate of 94% and a death rate of 16%. In the last decade more outbreaks have been caused by contaminated produce, and some of the produce has been processed. L. monocytogenes is a problem in food processing plants because of its ability to interact with stainless steel used in the processing equipment. In this study we tested a panel of 24 strains to assess their ability to grow attached to lettuce leaves and to stainless steel. Overnight growth of attached cells resulted in levels on lettuce from 0 – 3.4 log CFU/cm2, depending on the strain and the temperature. Among the worst performing strains on lettuce were two sourced from a large cantaloupe outbreak, indicating that factors important for interactions with cantaloupe may be different from those required on lettuce tissue. Strains that grew the best on lettuce belonged to various disease-relevant serotypes and were isolated from cheese, potatoes, and water/sediment near produce production fields. Microscopy of L. monocytogenes tagged with a constitutive Green Fluorescent Protein indicated associations with the cut edges and veins of lettuce leaves. For attached growth on stainless steel, strains of serotype 1/2a did better than strains of serotype 4b, which agrees with other studies. However, some serotype 4b strains grew well on the stainless steel, including strains isolated from water/sediment and from produce. Assessment of the performance of the strains in the different systems indicated strains that did well in both systems and strains that did well on stainless steel, but not on lettuce. While some similar molecular factors may be used for attached growth in both systems, it is likely that some factors are unique to either lettuce or stainless steel.

Technical Abstract: The foodborne pathogen Listeria monocytogenes lives as a saprophyte in nature and forms attachments and grows on surfaces as diverse as leaves, sediment, and stainless steel. To discern the mechanisms used by L. monocytogenes for attachment and growth on various surfaces, we studied interactions between the pathogen on lettuce and stainless steel. A panel of 24 strains (23 of Listeria monocytogenes and 1 L. innocua) was screened for attachment and growth on lettuce at 44C and 25oC and on stainless steel at 10oC and 37oC. Overnight growth of attached cells resulted in levels on lettuce from 0 – 3.4 log CFU/cm2, depending on the strain and the temperature. Among the worst performing strains on lettuce were two sourced from a large cantaloupe outbreak, indicating that factors important for interactions with cantaloupe may be different from those required on lettuce tissue. Strains that grew the best on lettuce belonged to serotypes 1/2a, 1/2b, and 4b and were isolated from cheese, potatoes, and water/sediment near produce production fields. Confocal microscopy of L. monocytogenes tagged with a constitutive Green Fluorescent Protein indicated associations with the cut edges and veins of lettuce leaves. On stainless steel coupons, growth of cells attached to stainless steel ranged from approximately 5 – 7 log CFU/cm2 at 10oC after 7 days and 4 – 7 log CFU/cm2 at 37oC after 40 hours. Statistically, surface growth on stainless steel was better for serotype 1/2a than for serotype 4b strains; however, some serotype 4b strains grew well while attached to the coupons. These included strains isolated originally from produce and from water/sediment. Some strains were fit in both environments, while others show variability between the different surfaces. Further analysis of these strains should reveal molecular factors needed for surface growth of L. monocytogenes on different biotic and abiotic surfaces.