Investigation of the hygienic safety of aromatic plants cultivated in soil contaminated with Listeria monocytogenes

The present work was undertaken to investigate the survival of Listeria monocytogenes ATCC 19114T insoil during the whole crop cycle of rocket (Eruca sativa Mill.) and basil (Ocimum basilicum L.), to monitorits transfer to the leaves, and to evaluate its viability at harvest. To this purpose, the soil was NePeKfertilized and four trials, obtained with different combinations of soil treatment, listerial inoculumsand seed planting, were followed for each aromatic plant. Soil was weekly investigated for total microbialcounts and L. monocytogenes evolution. At the starting time, un-inoculated autoclaved soil showeda limited microbial load (103 CFU g dw 1), while un-inoculated non-autoclaved soil contained approximately108 CFU g dw 1 microorganisms. Listerial persistence in inoculated soil was evaluated by platecounts and confirmed by randomly amplified polymorphic DNA (RAPD) analysis. Trials with nonautoclavedun-inoculated soil, used as control productions, contained about 104 CFU g dw 1 ofpresumptive Listeria spp., from the beginning till the end of the experimentation, which lasted six weeks.The trials artificially contaminated with approximately 109 CFU g dw 1 of L. monocytogenes showeda decrease of the initial inoculums, which was more rapid, reaching lower levels at harvest, for the trialswith non-autoclaved soil (4.95 and 4.81 log CFU g dw 1 for basil and rocket, respectively) than thosewith autoclaved soil (5.28 and 5.24 log CFU g dw 1 for basil and rocket, respectively). At harvest, plantsand soil samples were also analysed by denaturing gradient gel electrophoresis (DGGE). The last analysisshowed the presence of L. monocytogenes in soil, but not on the leaves of plants of all inoculated trials.