Factors affecting cell population density during enrichment and subsequent molecular detection of Salmonella enterica and Escherichia coli O157:H7 on lettuce contaminated during field production

- Growth of Escherichia and Salmonella was monitored during lettuce field production.
- Molecular detection of low populations of bacterial pathogens was evaluated.
- Detection of bacterial pathogens on leaves is challenged by environmental stress.
- Sample enrichment resulted a critical step for successful pathogen detection.

The aim of this study was to evaluate the field survival and subsequent detection of Escherichia coli O157:H7 and Salmonella on leafy greens to understand factors that influence their detection by molecular methods. Both applied microorganisms experienced a drop in population after 4 days of inoculation. and after 10 days, recovery was achieved only through enrichment. Field survival of microorganisms was affected by lettuce cultivar and better survival of Salmonella in comparison to E. coli O157:H7 was determined. Detection through real-time PCR was affected by duration of sample enrichment; shorter times (4-12 h) resulted in more false negatives, however after 18 h, detection was achieved in all samples. Red-pigmented lettuce cultivars caused inhibition of most PCR reactions. Preparation of tissue mass composites of 375 or 125 g in comparison to 25 g samples increased the rate of false negatives, the effect was pronounced when comparison was done between field-inoculated material compared to plant inoculated in the laboratory. In contrast, liquid compositing resulted a reliable strategy to reduce the number of molecular tests. Results highlighted the importance of specific commodity validation of molecular detection methods that consider sample matrix composition, natural microbiota and the cell stress of the pathogens.