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 in soil during the whole crop cycle of rocket (Eruca sativa Mill.) and basil (Ocimum basilicum L.), to monitor its transfer to the leaves, and to evaluate its viability at harvest. To this purpose, the soil was NePeK fertilized and four trials, obtained with different combinations of soil treatment, listerial inoculums and seed planting, were followed for each aromatic plant. Soil was weekly investigated for total microbial counts and L. monocytogenes evolution. At the starting time, un-inoculated autoclaved soil showed a limited microbial load (103 CFU g dw 1), while un-inoculated non-autoclaved soil contained approximately 108 CFU g dw 1 microorganisms. Listerial persistence in inoculated soil was evaluated by plate counts and confirmed by randomly amplified polymorphic DNA (RAPD) analysis. Trials with nonautoclaved un-inoculated soil, used as control productions, contained about 104 CFU g dw 1 of presumptive 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 showed a decrease of the initial inoculums, which was more rapid, reaching lower levels at harvest, for the trials with non-autoclaved soil (4.95 and 4.81 log CFU g dw 1 for basil and rocket, respectively) than those with autoclaved soil (5.28 and 5.24 log CFU g dw 1 for basil and rocket, respectively). At harvest, plants and soil samples were also analysed by denaturing gradient gel electrophoresis (DGGE). The last analysis showed the presence of L. monocytogenes in soil, but not on the leaves of plants of all inoculated trials.