Listeria monocytogenes Contamination Leads to Survival and Growth During Enoki Mushroom Cultivation.

Two recent outbreaks of listeriosis have been linked to the consumption of enoki mushrooms. After the first outbreak, import sampling by the U.S. FDA identified that 43% of the samples evaluated were positive for Listeria monocytogenes (Lm). These observations raised questions about the potential sources of Lm contamination of enoki mushrooms. One potential source of contamination is during enoki mushroom cultivation, as growing conditions are comparatively cool and moist to induce mushroom germination, to which Lm is well adapted. Two varieties of enoki mushrooms were evaluated to determine the potential for Lm to contaminate enoki cultures when introduced at various points during cultivation (inoculation, scraping, pinning, and collaring). The results of two trials showed that Lm established contamination and grew to similar levels in the substrate regardless of when Lm was introduced and, with one exception, did not alter the rate of mushroom generation to below the control. Enumeration of Lm in enoki mushroom cultures at harvest found an average contamination of 103 cfu/g, though the results were variable. Refrigerated storage for six weeks was found to result in an increase in Lm. Additionally, no statistically significant difference in the levels of Lm was observed based on proximity to the substrate, though levels of Lm in the different enoki samples correlated with levels of Lm in the substrate at harvest, but not at scraping. The ability of Lm to grow independently in the media used to culture enoki was assessed, and Lm was found to be unable to grow but could sporadically survive in Masters Mix. No growth of Lm was observed in potato dextrose broth, though growth could occur on the agar. Overall, the data indicate a high potential for the establishment of Lm contamination at any point during enoki cultivation to result in Lm-contaminated mushrooms. These data indicate a need for active control mechanisms to prevent the introduction of Lm to enoki cultures.

Development and Evaluation/Verification of a Fully Automated Test Platform for the Rapid Detection of Cyclospora cayetanensis in Produce Matrices.

Cyclosporiasis, caused by the coccidian parasite Cyclospora cayetanensis, has emerged as an increasing global public health concern, with the incidence of laboratory-confirmed domestically acquired cases in the US exceeding 10,000 since 2018. A recently published qPCR assay (Mit1C) based on a mitochondrial target gene showed high specificity and good sensitivity for the detection of C. cayetanensis in fresh produce. The present study shows the integration and verification of the same mitochondrial target into a fully automated and streamlined platform that performs DNA isolation, PCR, hybridization, results visualization, and reporting of results to simplify and reduce hands-on time for the detection of this parasite. By using the same primer sets for both the target of interest (i.e., Mit1C) and the internal assay control (IAC), we were able to rapidly migrate the previously developed Mit1C qPCR assay into the more streamlined and automated format Rheonix C. cayetanensisTM Assay. Once the best conditions for detection were optimized and the migration to the fully automated format was completed, we compared the performance of the automated platform against the original "bench top" Mit1C qPCR assay. The automated Rheonix C. cayetanensis Assay achieved equivalent performance characteristics as the original assay, including the same performance for both inclusion and exclusion panels, and it was able to detect as low as 5 C. cayetanensis oocysts in fresh produce while significantly reducing hands-on time. We expect that the streamlined assay can be used as a tool for outbreak and/or surveillance activities to detect the presence of C. cayetanensis in produce samples.

Development and Single Laboratory Evaluation of a Refined and specific Real-time PCR Detection Method, Using Mitochondrial Primers (Mit1C), for the Detection of Cyclospora cayetanensis in Produce.

Regulatory methods for detection of the foodborne protozoan parasite Cyclospora cayetanensis must be specific and sensitive. To that end, we designed and evaluated (in a single laboratory validation) a novel and improved primer/probe combination (Mit1C) for real-time PCR detection of C. cayetanensis in produce. The newly developed primer/probe combination targets a conserved region of the mitochondrial genome of C. cayetanensis that varies in other closely related organisms. The primer/probe combination was evaluated both in silico and using several real-time PCR kits and polymerases against an inclusivity/exclusivity panel comprised of a variety of C. cayetanensis oocysts, as well as DNA from other related Cyclospora spp. and closely related parasites. The new primer/probe combination amplified only C. cayetanensis, thus demonstrating specificity. Sensitivity was evaluated by artificially contaminating cilantro, raspberries, and romaine lettuce with variable numbers (200 and 5) of C. cayetanensis oocysts. As few as 5 oocysts were detected in 75%, 67.7%, and 50% of the spiked produce samples (cilantro, raspberries, and romaine lettuce), respectively, all uninoculated samples and no-template real-time PCR controls were negative. The improved primer/probe combination should prove an effective analytical tool for the specific detection of C. cayetanensis in produce.

Evaluation of the taxonomic utility of six-enzyme pulsed-field gel electrophoresis in reconstructing Salmonella subspecies phylogeny.

Pulsed-field gel electrophoresis (PFGE) remains an important tool in the molecular epidemiological evaluation of strains emerging from disease outbreak clusters. Recent studies of Escherichia coli O157:H7 and Salmonella Enteritidis have noted marked improvements in the discriminatory power of PFGE when combining band profiles from up to six restriction enzyme datasets into a single concatenated analysis. This approach has provided more accurate assignments of genetic relationships among closely related strains and allowed effective phylogenetic inference of host and geographical reservoirs. Although this approach enhances epidemiological congruence among closely related strains, it remains unclear to what extent six-enzyme PFGE pattern similarity reiterates evolutionary relatedness among more distantly related Salmonella strains (i.e., serovar or subspecies levels). Here, taxonomic accuracy of six-enzyme PFGE is tested phylogenetically across two distinct Salmonella enterica populations-Salmonella reference collection B (SARB), representing the breadth of taxonomic diversity of S. enterica subspecies I only, and Salmonella reference collection C (SARC), comprising the seven disparate subspecies of S. enterica plus S. bongori. Cladistic analysis of SAR strains revealed substantial polyphyly between the two strain collections such that numerous SARB strains clustered more closely with diverged SARC subspecies rather than with other members of subspecies I. Also, in several cases, SARC sibling strains from the same subspecies were evolutionary obscured-broken into distant locales on the most parsimonious six-enzyme trees. Genetic diversity among SARB and SARC strains was comparable at 45% and 47%, respectively, while congruence testing revealed discordance among individual enzyme datasets. While six-enzyme PFGE is effective in ascertaining accurate genetic relationships for more closely related strains (e.g., strains within the same serovar), reconstitution of evolutionarily meaningful strain groupings may be elusive for Salmonella at the serovar level and above. Thus, caution is warranted when applying PFGE with a limited number of enzymes as the primary phylogenetic marker in these instances.