Distribution, contamination routes, and seasonal influence of persistent Listeria monocytogenes in a commercial fresh Hypsizigus marmoreus production facility

Abstract Listeria monocytogenes is an important foodborne pathogen worldwide, with a high mortality rate of 20%-30% for vulnerable persons. The aim of this study was to determine the distribution, contamination routes, and persistence of L. monocytogenes in a mushroom plant. A total of 204 samples collected during four sampling periods within a one-year time interval, with 27.0% (55/204) were positive for L. monocytogenes. A total of 60 representative L. monocytogenes strains were selected for further analysis based on serogrouping and enterobacterial repetitive intergenic consensus polymerase chain reaction (ERIC-PCR) fingerprints. Multiplex PCR showed serogroups I.1 (1/2a-3a) and II.2 (1/2b-3b-7) accounted for 68.3% (41/60) and 31.7% (19/60), respectively, of the samples. ST8 strains were isolated from the mycelium-scraping phase to the harvest phase by multilocus sequence typing. These strains had identical ERIC-PCR fingerprints, suggesting that the mycelium-scraping machine was the main contamination source. Sequence types (ST) 8 (61.7%, 37/60) and 87 (23.3%, 14/60) were predominant and repeatedly isolated within a one-year seasonal investigation, indicating that ST8 and ST87 persist in this plant. The minimum inhibitory concentrations of commonly used quaternary ammonium compounds for the 60 isolates were ≥ 4 μg/mL, indicating that the persistence we observed may be associated with biocide tolerance. In addition, all strains harbored nine virulent genes (prfA, hly, mpl, actA, iap, plcA, plcB, inlA, and inlB). Only ST87 strains carried ptsA (representing LIPI-4), but none of the strains harbored llsX (representing LIPI-3). These results indicate that ST87 strains pose a potential public risk to consumers. All isolates were susceptible to 13 antimicrobials, except for levofloxacin (1.7%, 1/60), erythromycin (3.3%, 2/60), and clindamycin (88.3%, 53/60). This study is the first to explore the contamination routes and persistence of L. monocytogenes in a Hypsizigus marmoreus production facility. Critical disinfection measures should be adopted to eliminate its persistence throughout processing.