Lettuce is overrated: Bad bugs in leafy greens

Microbial pathogen infiltration in fresh leafy greens is a significant food safety risk factor.

lettuce.tomato.skullIn various postharvest operations, vacuum cooling is a critical process for maintaining the quality of fresh produce. The overall goal of this study was to evaluate the risk of vacuum cooling-induced infiltration of Escherichia coliO157:H7 into lettuce using multiphoton microscopy.

Multiphoton imaging was chosen as the method to locate E. coli O157:H7 within an intact lettuce leaf due to its high spatial resolution, low background fluorescence, and near-infrared (NIR) excitation source compared to those of conventional confocal microscopy. The variables vacuum cooling, surface moisture, and leaf side were evaluated in a three-way factorial study with E. coli O157:H7 on lettuce. A total of 188 image stacks were collected. The images were analyzed for E. coli O157:H7 association with stomata and E. coli O157:H7 infiltration. The quantitative imaging data were statistically analyzed using analysis of variance (ANOVA).

The results indicate that the low-moisture condition led to an increased risk of microbial association with stomata (P < 0.05). Additionally, the interaction between vacuum cooling levels and moisture levels led to an increased risk of infiltration (P < 0.05). This study also demonstrates the potential of multiphoton imaging for improving sensitivity and resolution of imaging-based measurements of microbial interactions with intact leaf structures, including infiltration.

 Influence of vacuum cooling on Escherichia coli O157:H7 infiltration infresh leafy greens via a multiphoton-imaging approach

Appl. Environ. Microbiol. January 2016 82:106-115; Accepted manuscript posted online 16 October 2015, doi:10.1128/AEM.02327-15

Erica Vonasek and Nitin Nitin