Foliar Roughness and Water Content Impact on Escherichia coli Attachment in Baby Leafy Greens

SIMPLE SUMMARY: Vegetables may host human pathogens, such as Salmonella enterica and Escherichia coli. Therefore, the consumption of salads may raise safety concerns as they are eaten raw and often without prior proper washing. Bringing to light the differences between different salad crops in the susceptibility to human pathogen contamination and understanding the relationship between the susceptibility and leaf traits can contribute to increase food safety. We evaluated the susceptibility to E. coli attachment of 30 different baby leaves (leafy vegetables harvested at an early stage of growth and consumed as salads), finding inter- and intraspecific variation. In romaine lettuce, the most involved in food disease outbreaks in the U.S., we found genotypes (‘Maraichere’) less susceptible to contamination than others (‘Bionda degli Ortolani’). Among the 30 baby leaves, ‘Bionda degli Ortolani’, rocket and Swiss chard were the most susceptible, with wild rocket, wild lettuce and lamb’s lettuce the least. Further analysis of these six baby leaves revealed that leaf roughness and water content were positively correlated with attachment level. In rougher leaves, we observed a lower effectiveness of UV treatment in reducing attachment. ABSTRACT: Understanding the relation between the susceptibility of different leafy greens to human pathogen contamination and leaf traits can contribute to increase the food safety of the fresh vegetable industry. The aim of this research was to evaluate the susceptibility to E. coli ATCC 35218 attachment in 30 accessions of baby leaves, and to identify leaf traits potentially involved in the contamination. The accessions were surface inoculated with a bacterial suspension containing 1 × 10(7) cells/mL and the attachment was measured 1.5 h after inoculation. Significant differences in attachment were detected between the accessions for p ≤ 0.05. The three most and the three least susceptible accessions were selected and characterized for leaf micro-morphological traits (stomata density and size, surface roughness) and water content. Scanning electron microscopy was used to analyse the stomatal parameters. Roughness was measured by an innovative portable 3D digital microscope. No significant correlation between the attachment of E. coli ATCC 35218 and stomatal parameters was detected, while the attachment was positively correlated with roughness and water content. The E. coli ATCC 35218 population in surface-inoculated leaves was also measured after a UV treatment, which was found to be less effective in reducing bacterial contamination in the rougher leaves. This result suggested that roughness offers UV protection, further highlighting its impact on the microbiological safety of baby leafy greens.