Contamination pathways of Listeria monocytogenes in Enoki mushrooms and their production environment at a commercial farm in South Korea

Abstract

Listeria monocytogenes is a significant foodborne pathogen associated with high morbidity and mortality, particularly in immunocompromised populations. Contamination of Listeria monocytogenes in enoki mushrooms has led to multiple international outbreaks in recent years, yet the precise pathways of contamination during production remain largely uninvestigated. This study is the first to comprehensively examine the contamination dynamics of L. monocytogenes within a commercial enoki mushroom production facility in South Korea. Notably, our study focused on critical control points and molecular characteristics of the isolates to identify potential cross-contamination points throughout the production process. In total, 2088 samples were collected across various production stages, equipment, and products. L. monocytogenes was identified using cultural methods, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), and polymerase chain reaction (PCR). Serotyping and pulsed-field gel electrophoresis (PFGE) were employed to assess genetic diversity and distribution. Twenty-six isolates (1.12 %) were recovered, with the highest prevalence observed at the fungus-scraping stage (4.67 %). Floor swabs exhibited the highest contamination rate (40 %), and 10 isolates were recovered from blender blades at this stage. No isolates were detected before this fungus-scraping stage, suggesting it was the primary point of contamination. Serotype 1/2b was predominant (96 %), with a single isolate identified as serotype 1/2a. PFGE revealed six distinct pulsotypes, with one dominant pulsotype (I) distributed across multiple stages and surfaces, indicating persistent contamination and potential biofilm formation. Two isolates (LM21 and LM23), recovered from a cutting knife and a manually packaged mushroom, shared identical PFGE patterns, suggesting cross-contamination. These findings underscore the fungus-scraping stage as a critical point and emphasize the need for enhanced sanitation protocols and targeted interventions to ensure microbiological safety in enoki mushroom production.