Vegetables are major components of healthy and balanced diet. However, 25% of foodborne
diseases are linked to the consumption of vegetables, especially the minimally processed ready to
eat (RTE) vegetables. The main foodborne pathogens associated at RTE vegetables are
Enterobacteriaceae (E. coli, Salmonella spp. and Yersinia spp.) and psychrophilic microorganisms
like Listeria monocytogenes.
Aim of study
The aim of this work was to assess the microbiological risks associated with consumption of RTE
salads, through the quantification of microbiological contamination. In addition we carried out the
rapid detection of foodborne pathogens by real time PCR, and molecular characterization of Listeria
monocytogenes strain isolated in this study. Microbiological and bio molecular challenge tests was
carried out for assess potential growth of Listeria monocytogenes in vegetables stored at different
temperatures. Besides, the work was focused on the evaluation of washing process in association to
the shelf life of these products.
Materials and methods
A total of 300 pre-packaged mixed raw vegetable salads collected from retail premises (68%) and
production plants (32%) were examined. Microbiological eligibility for human consumption (Reg
CE 1441/2007) is based on three bacteriological parameters (Escherichia coli, Salmonella spp and
L. monocytogenes). In order to assess the safety of RTE vegetables, all parameters required by law
have been investigated. The contamination of vegetables from E. coli O157:H7, L. monocytogenes
and Salmonella spp was also investigated by real time PCR. Furthermore, the pathogenic properties
of L. monocytogenes have been evaluated by PCR assay (prfA, rrn, hlyA, actA, inlA, inlB, iap, plcA
e plcB). The purpose of Challenge test was to provide information especially on the behavior of L.
Results and conclusion
Parsley and mixed salads showed the most contaminated panel, however we found high levels of
contamination by Enterobacteriaceae in every kind of RTE salads. Salmonella spp. and E. coli
O157:H7 have never been isolated, while only a sample of rocket was contaminated by one strain of
L. monocytogenes and Yersinia enterocolitica. The PCR assay showed the virulence genes of L.
monocytogenes strain (prfA, rrn, actA, inlA, inlB, plcA, plcB). Washing temperature,
microbiological quality of water and raw material are basic requirements to achieve high
microbiological standards for RTE salads. Listeria monocytogenes shows a growth potential
variable in consideration of the different types of products and storage temperatures. Bio molecular
challenge tests appear a useful tool for the evaluation of the survival of L. monocytogenes when
coltural results seem uncertain (eg. high or low bacterial growth and difficulty in estimating the