Survival of Listeria monocytogenes biofilms and planktonic cells on cooked ham in a contamination-storage-digestion process

In 2023, listeriosis experienced a significant increase in Europe, with 2952 invasive cases and 19.70 % mortality rate, being the ready-to-eat foods category identified as the mostly involved. This study focused on the survival of L. monocytogenes through a process of food contamination-storage-digestion. For this, cooked ham was artificially contaminated with biofilm or planktonic cells, followed or not by vacuum-packed storage at 7 °C for 7 days, and submitted to in vitro digestion. Enumeration analysis was performed at different stages during the contamination-storage-digestion process. Contamination of cooked ham by 90-s transfer from biofilm revealed inter-strain variability, with a global transfer rate of 14.07 %. On the food, L. monocytogenes grew ≈0.5 log during storage from an initial concentration of 3.53 log CFU/cm². In vitro digestion of contaminated food with or without storage, resulted in a reduction of ≈−0.5 log. In the case of planktonic cells, great differences were found after in vitro digestion as a function of storage (≈−2.1 log or ≈−0.6 log reduction from the initial concentration without or with storage, respectively). Interestingly, an unexpectedly high concentration of the pathogen was found after food storage and digestion. It is plausible that a biofilm was formed during storage of the contaminated food with planktonic cells. In any case, high variability in numbers was observed after storage and digestion, being L196 the most resistant strain. More research in omics sciences is needed to clarify L. monocytogenes behaviour.