Listeria monocytogenes and Listeria spp. Contamination Patterns in Retail Delicatessen Establishments in Three U.S. States

Abstract

Postprocessing contamination in processing plants has historically been a significant source of Listeria monocytogenes in ready-to-eat delicatessen meats, and therefore a major cause of human listeriosis cases and outbreaks. Recent risk assessments suggest that a majority of human listeriosis cases linked to consumption of contaminated deli meats may be due to L. monocytogenes contamination that occurs at the retail level. To better understand the ecology and transmission of Listeria spp. in retail delicatessens, food and nonfood contact surfaces were tested for L. monocytogenes and other Listeria spp. in a longitudinal study conducted in 30 retail delis in three U.S. states. In phase I of the study, seven sponge samples were collected monthly for 3 months in 15 delis (5 delis per state) prior to start of daily operation; in phase II, 28 food contact and nonfood contact sites were sampled in each of 30 delis during daily operation for 6 months. Among the 314 samples collected during phase I, 6.8% were positive for L. monocytogenes. Among 4,503 samples collected during phase II, 9.5% were positive for L. monocytogenes; 9 of 30 delis showed low L. monocytogenes prevalence (<1%) for all surfaces. A total of 245 Listeria spp. isolates, including 184 Listeria innocua, 48 Listeria seeligeri, and 13 Listeria welshimeri were characterized. Pulsed-field gel electrophoresis (PFGE) was used to characterize 446 L. monocytogenes isolates. PFGE showed that for 12 of 30 delis, one or more PFGE types were isolated on at least three separate occasions, providing evidence for persistence of a given L. monocytogenes subtype in the delis. For some delis, PFGE patterns for isolates from nonfood contact surfaces were distinct from patterns for occasional food contact surface isolates, suggesting limited cross-contamination between these sites in some delis. This study provides longitudinal data on L. monocytogenes contamination patterns in retail delis, which should facilitate further development of control strategies in retail delis.

Whole-Genome Sequencing Allows for Improved Identification of Persistent Listeria monocytogenes in Food-Associated Environments

“…Here, we found that three sets of PFGE types that differed by three or fewer bands differed in the presence of an intact prophage regions; these findings clarified that a single L. monocytogenes clone presented by two PFGE types can persist in a deli. For example, we previously reported reisolation of two PFGE types (CU-11-320 and CU-8-96) in deli 2 (20); WGS showed that these two PFGE types represent one clone with a MRCA of Ͻ7 years.…”

“…monocytogenes contamination at retail represents a complicated scenario with a mixture of persistence and repeated introduction from external sources. A key goal of the present study was to apply WGS-based subtyping to better understand the sources and transmission of L. monocytogenes in retail delis, building on a previous PFGE-based study of L. monocytogenes isolates obtained during a longitudinal study of 30 retail delis in three states (20). A specific goal was to use WGS-based subtyping to differentiate persistent L. monocytogenes contamination in delis from contamination that likely represents repeated introduction from an external source.…”

“…For example, three PFGE types (CU-11-320, CU-262-79, and CU-8-96) were repeatedly isolated in multiple delis (four, three, and two delis, respectively [20]). In a number of cases, the WGS-SNP data reported here resolved isolates with a PFGE type into a number of well-supported clades that (i) included only isolates from a single deli and (ii) likely diverged from a most recent common ancestor less than 10 years ago.…”

“…SRST2 was used to assign sigB allelic types to each genome sequence. If the sigB allelic type for a genome sequence did not match an isolate's sigB type previously determined by Sanger sequencing (20), then this isolate was excluded from analysis (two isolates were excluded: FSL R8-5744 and FSL R8-5866). In addition, isolate FSL N3-0993 was excluded because the genetic lineage of the sequenced isolate (lineage II) did not match the previously determined genetic lineage for the subtype determined from PCR serogrouping (the isolate was previously assigned to serotypes 1/2b, 3b, or 7 based on reference 36, a finding consistent with genetic lineage I [37]).…”

Genes Associated with Desiccation and Osmotic Stress in Listeria monocytogenes as Revealed by Insertional Mutagenesis

“…Recent literature also suggests that contamination at the retail level may be a significant cause of listeriosis (9)(10)(11). In both environments, L. monocytogenes has been isolated from food contact surfaces and non-food contact surfaces, such as floors and drains (12).…”