Listeria monocytogenes has emerged as a food safety concern for several produce commodities. Although L. monocytogenes contamination can occur throughout the supply chain, contamination from the packinghouse environment represents a particular challenge and has been linked to outbreaks and recalls. This study aimed to investigate the prevalence, persistence, and diversity of L. monocytogenes and other species of Listeria in produce packinghouses. A longitudinal study was performed in 11 packinghouses (whose commodities included microgreen, peach, apple, tomato, broccoli, cauliflower, and cucumber) in three U.S. states. In each packinghouse, 34 to 47 sites representing zones 2 to 4 were selected and swabbed. Packinghouses were visited four times over the packing season, and samples were tested for Listeria by following the U.S. Food and Drug Administration's Bacteriological Analytical Manual methods. Presumptive Listeria-positive isolates were confirmed by PCR. Species and allelic type (AT) were identified by sigB sequencing for up to eight isolates per sample. Among 1,588 samples tested, 50 (3.2%), 42 (2.7%), and 10 (0.6%) samples were positive for L. monocytogenes only, Listeria spp. (excluding L. monocytogenes) only, and both L. monocytogenes and Listeria spp., respectively. Five species of Listeria (L. monocytogenes, L. innocua, L. seeligeri, L. welshimeri, and L. marthii) were identified, and L. monocytogenes was the most prevalent species. The 102 Listeria-positive samples yielded 128 representative isolates (i.e., defined as isolates from a given sample with a different AT). Approximately 21% (21 of 102) of the Listeria-positive samples contained two or more ATs. A high AT diversity (0.95 Simpson's diversity index) was observed among Listeria isolates. There were three cases of L. monocytogenes or Listeria spp. repeated isolation (site testing positive at least twice) based on AT data. Data from this study also support the importance of drain and moisture management, because Listeria were most prevalent in samples collected from drain, cold storage, and wet nonfood contact surface sites. HIGHLIGHTS

The 2014 caramel apple listeriosis outbreak was traced back to cross-contamination between food contact surfaces (FCS) of equipment used for packing and fresh apples. For Washington State, the leading apple producer in the U.S with 79% of its total production directed to the fresh market, managing the risk of apple contamination with Listeria monocytogenes within the packing environment is crucial. The objectives of this study were to determine the prevalence of Listeria spp. on FCS in Washington State apple packinghouses over two packing seasons, and to identify those FCS types with the greatest likelihood to harbor Listeria spp. Five commercial apple packinghouses were visited quarterly over two consecutive year-long packing seasons. A range of 27 to 50 FCS were swabbed at each facility to detect Listeria spp. at two timings of sampling, (i) post-sanitation and (ii) in-process (three hours of packinghouse operation), following a modified protocol of the FDA’s Bacteriological Analytical Manual method. Among 2,988 samples tested, 4.6% (n=136) were positive for Listeria spp. Wax coating was the unit operation from which Listeria spp. were most frequently isolated. The FCS that showed the greatest prevalence of Listeria spp. were polishing brushes, stainless steel dividers and brushes under fans/blowers, and dryer rollers. The prevalence of Listeria spp. on FCS increased throughout apple storage time. The results of this study will aid apple packers in controlling for contamination and harborage of L. monocytogenes and improving cleaning and sanitation practices of the most Listeria-prevalent FCS. IMPORTANCE Since 2014, fresh apples have been linked to outbreaks and recalls associated with post-harvest cross-contamination with the foodborne pathogen L. monocytogenes. These situations drive both public health burden and economic loss and underscore the need for continued scrutiny of packinghouse management to eliminate potential Listeria spp. niches. This research assesses the prevalence of Listeria spp. on FCS in apple packinghouses and identifies those FCS most likely to harbor Listeria spp. Such findings are essential for the apple packing industry striving to further understand and exhaustively mitigate the risk of contamination with L. monocytogenes to prevent future listeriosis outbreaks and recalls.

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Flies are a vector for spreading foodborne pathogens pertinent to fresh produce, such as Shiga-toxigenic Escherichia coli and Salmonella; however, most studies focus on concentrated animal feeding operations, which do not reflect low-density animal farming practices that often adjoin fruit and vegetable acreage.  In this study, we determined the prevalence of Salmonella in flies collected biweekly on an integrated animal and produce operation over two growing seasons. Eleven pooled samples, out of 889, tested positive for Salmonella. Flies from the Calliphoridae, Muscidae, Sarcophagidae, and Tachinidae families were associated with Salmonella carriage, but fly family was not a significant factor for isolation of Salmonella (p=0.303). Fly species were a significant factor (p=0.026), with five Pentacricia aldrichii pools testing positive for Salmonella. With the exception of single specimen isolation, prevalence ranged from 2.2 to 15.2%. With the exception of the Tachinidae family, these results reflect a strong association of flies that are commonly associated with feces or are pests of animals. Trap location was not significantly associated with isolation of Salmonella-positive flies (p=0.236). Overall, the population of flies was not as abundant as studies conducted with produce grown in close proximity to concentrated animal feeding operations, indicating a reduced risk of transmission; however, similar to these studies, fly families that are commonly isolated from fecal and decaying matter were most frequently associated with Salmonella isolation. Further work is warranted to elucidate the foodborne pathogen transmission rates to produce and subsequent survival over time.

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