Tidsskrift: PLOS ONE, vol. 16, p. 22, 2021
Open Access: gold
he frequency of foodborne outbreaks epidemiologically associated with Listeria monocytogenes in fresh produce has increased in recent years. Although L. monocytogenes may be
transferred from the environment to vegetables during farming, contamination of food products most commonly occurs in food processing facilities, where L. monocytogenes has the
ability to establish and persist on processing equipment. The current study was undertaken
to collect data on the occurrence of L. monocytogenes and the identity of the endogenous
microbiota in a fresh produce processing facility, for which information has remained scarce.
L. monocytogenes was not detected in the facility. Experiments simulating conditions in the
processing environment were performed, including examination of bacterial growth in nutrients based on vegetables (salad juice) compared to in other types of nutrients (fish, meat).
Results showed that the endogenous microbiota (dominated by Pseudomonas) grew well in
iceberg lettuce and rocket salad juice at low temperatures, while growth inhibition of L. monocytogenes was observed, particularly in rocket salad juice. The anti-listerial activity in rocket
salad juice was retained in a polar chromatographic fraction containing several metabolites.
Characterization of this active fraction, using LC-MS/MS, led to identification of 19 compounds including nucleosides and amino acids. Further work is necessary to determine the
molecular mechanism responsible for the inhibitory activity of rocket salad constituents. The
study nevertheless suggests that the available nutrients, as well as a low temperature (3 ˚C)
and the in-house bacterial flora, may influence the prevalence of L. monocytogenes in fresh
produce processing facilities.