Foodborne illness in the United States continues to be a complex and recurring issue despite our increased understanding of the pathogenic microorganisms responsible. Foodborne illness outbreaks and product recalls linked to pathogenic bacteria have been more frequent in the produce industry (e.g. fruits and vegetables) in the last ten years. Currently one of the most concerning foodborne bacterial pathogens in the produce industry is Listeria monocytogenes. This foodborne pathogen has been linked to multistate produce-associated outbreaks causing hundreds of illnesses and dozens of deaths. In several of these outbreaks, L. monocytogenes strains isolated from clinical patients were found to be persistent in produce handling and processing (PHP) facilities. This suggests that L. monocytogenes may contaminate product through cross-contamination events in PHP facilities and current food safety interventions in these environments may be inadequate to prevent transfer to food.
Since the passage of the Food Safety and Modernization Act (FSMA) in 2011, States in the Pacific Northwest (PNW) that supply the U.S. with hundreds of specialty crops have been more focused on food safety. The produce industry in the PNW needs data and knowledge to effectively control L. monocytogenes in PHP facilities and comply with FSMA. The objective of this study was to investigate the prevalence of Listeria spp. in seven PHP facilities in Oregon and Washington through environmental monitoring on non-food contact surfaces only, with emphasis on the pathogenic species L. monocytogenes. The facility with the highest prevalence would receive additional and more intensive environmental sampling. A secondary objective was to characterize Listeria spp. strains recovered from PHP facilities and group potentially related strains. Characterization of strains was done through speciation, a multiplex PCR serogrouping assay, and antimicrobial resistance (AMR) profiling. A third objective was to track related strains throughout one facility and identify potential contamination sources.
Environmental samples were collected from all PHP facilities at least twice (Rounds 1 and 2) from 2018-2019 and tested for Listeria spp. using a modified ISO 11290-1 method. Listeria spp. were not recovered from two PHP facilities (5/7, 70%). The prevalence of Listeria spp. through the first two rounds varied significantly across all PHP facilities and overall prevalence was relatively low (24/350, 6.9%). Additionally, L. monocytogenes was recovered in all PHP facilities positive for Listeria spp. One facility contributed >50% of the positive samples for the entire study. This facility minimally processes and packs raw produce only, does not have an environmental monitoring program and it is not subject to environmental monitoring regulations included in FSMA.
Throughout the next rounds of sampling in only this facility (Rounds C and D)
L. monocytogenes was more frequently recovered from environmental samples (26/100, 26%). A majority of L. monocytogenes strains were recovered from production room drains, foot traffic and forklift traffic entry points, samples taken outside the facility and in high traffic production floor areas. Characterization and tracking suggested that Listeria spp. are commonly brought into this facility on the bottom of employee shoes or forklifts and subsequently deposited throughout the facility.
Serogrouping of L. monocytogenes strains showed that isolates from all facilities may be serotypes that are regularly associated with listeriosis foodborne illness outbreaks, serotypes 1/2a and 4b. AMR profiling, though, indicated that all recovered Listeria spp. strains were sensitive to antibiotics commonly used in the treatment of foodborne listeriosis. Collectively, our data suggest that there is an increased risk of environmental contamination for PHP facilities that function as packinghouses and handle multiple types of raw produce, though additional studies including diverse PNW PHF facilities are needed to support this hypothesis. Antibiotic resistance in L. monocytogenes food chain isolates should be continuously monitored, including further genomic characterization of isolates to better understand overall strain relatedness, pathogenicity and AMR potential.