Effect of Contamination Matrices on the Persistence of Escherichia coli O157:H7 on Romaine Lettuce Leaves

Introduction: Produce outbreaks that are ultimately traced to pre-harvest farming environments are frequently associated with either direct or indirect contact with animal manure. Despite the numerous matrices found on farm in which foodborne pathogens can survive and in some cases thrive (e.g., compost, manure, soil, water, produce extracts), much sanitizer and surfactant efficacy research is still being performed using either water or pH-buffered saline as the inoculation matrix. This discrepancy between real-world contamination scenarios and laboratory practices has, in some cases, led to the selection of ineffective sanitizing solutions in post-harvest processing of leafy greens.

Purpose: This study examined the effect of growing (and inoculating) E. coliO157:H7 in various matrices on its persistence on Romaine lettuce leaves during plant growth and resistance to post-harvest processing.

Methods: Escherichia coli O157:H7 cells were inoculated into various matrices (irrigation water, soil, cattle manure or produce extracts). Inocula matrices were applied to the leaves of Romaine lettuce grown in BSL-2 growth chambers at 0, 2, 4, 8, and 16 days prior to harvesting to simulate contamination occurring during produce growing. Lettuce leaves were harvested, stored for 0 to 3 days at 5°C to simulate pre-processing storage, then exposed via simulated commercial washing to various disinfection treatments. Microbial enumeration before and after treatments included use of standard spread-plating and MPN methodologies.

Results: The contamination matrix had a significant impact on pathogen retention and subsequent removal/inactivation. A 4-log reduction of E. coli O157:H7 was achieved when the source was a contaminated irrigation water matrix. Lower, 2-log reductions were determined for dairy manure, dairy manure extract and soil which all showed variable responses to chlorine wash reductions, depending on leaf-surface morphology and hydrophobicity.

Significance: Sanitization efficacy tests should be conducted with inoculation matrices mimicking real-life contamination scenarios.