Purpose: To monitor the population dynamics of low levels of E. coli O157:H7 and S. entericaon the phyllosphere of diverse mini-greens during field production and identify factors that influence detection after environmental exposure.
Methods: Five varieties of lettuce were inoculated with two inoculum doses (log 4 and log 6 CFU/ml) of attenuated strains of E. coli O157:H7 (PTVS155) and S. enterica sv. Typhimurium (PTVS177) during field production. Changes in population were monitored after 12 h, 3 and 10 days post inoculation using selective quantification and/or enrichment. Additionally, molecular detection with commercial kits using different sample sizes (25 to 375 g), enrichment media composition and ratio, and time of enrichment in field inoculated samples, as well as in vitroinoculated lettuce samples, were compared.
Results: Populations of E. coli O157:H7 and S. enterica significantly declined 12 h post inoculation; surviving populations were recovered after selective enrichment. Prevalence of both pathogens was reduced with the time of exposure in the open environment. Overall, better fitness on the phyllosphere was observed for S. enterica than the applied E. coli O157:H7. Detection assays demonstrated that large sample mass (375 g) in combination with low ratio (plant material:enrichment broth), short enrichment periods (<12 h), and non-green pigmentation of leaves increased the occurrence of false negatives. A lower percentage of positive samples was detected in field-inoculated than in vitroinoculated samples of the same leaf tissues.
Significance: Validation of molecular detection platforms for pathogens on fresh produce should be done in a context that considers abiotic and biotic phyllosphere interactions on the target pathogen as well as sample mass and processing matrix.