Listeria monocytogenes occurrence and genetic diversity within the pre-packaging area of an Italian cold-smoked salmon processing plant

Listeria monocytogenes is a major concern in ready-to-eat (RTE) food produc- tion, particularly in cold-smoked salmon, which has been linked to several lis- teriosis outbreaks. This study investigated the occurrence and genetic diversity of L. monocytogenes within the pre-packaging area of an Italian cold-smoked salmon processing plant. From March 2023 to December 2023, 69 samples were collected, including 44 food contact surfaces (FCS), 15 non-food contact surfaces (NFCS), and 10 food samples (F). Microbiological analyses were con- ducted according to the standard qualitative method EN ISO 11290:2017, and suspected colonies were confirmed by MALDI-TOF mass spectrometry. Isolates were further characterized by multilocus sequence typing (MLST). As results, 18 samples showed the presence of L. monocytogenes, corresponding to an overall detection rate of 26.09%. The highest positivity rate was observed among NFCS (86.67%), followed by F (10%) and FCS (9.09%). MLST grouped isolates into five STs: ST5 (38.89%), ST9 (27.78%), ST6 (16.67%), ST7 (11.11%), ST14 (5.56%). ST5 and ST9 were the predominant STs and were repeatedly detected throughout the sampling period. These findings highlighted the importance of environmental monitoring, strengthened sanitation measures, and molecular typing to improve L. monocytogenes control in cold-smoked salmon processing plants.