Epiphytic, Attached, and Internal Escherichia coli O157:H7 Subpopulations Associating With Romaine Lettuce Are Strain‐Dependent and Affected by Relative Humidity and Pre‐ and Postharvest Plant State

ABSTRACT Romaine lettuce is susceptible to Escherichia coli O157:H7 contamination. We evaluated strain and pre‐ and postharvest lettuce product differences in E. coli O157:H7 subpopulation distribution on romaine lettuce at two relative humidity (RH) levels. Plants of romaine lettuce cultivar 'Carlsbad' harvested and processed 'Carlsbad' leaves, and store‐bought ready‐to‐eat romaine lettuce were inoculated with E. coli O157:H7 reference strain EDL933 and romaine lettuce outbreak strain 2705C. Using four processing methods, we determined pathogen cell fractions representing All (entire population), Epiphytic (loosely attached cells), Strongly Attached + Internal (excluding loosely attached cells), and Internal (excluding epiphytic cells) subpopulations. Preharvest, 80% RH favored subpopulations in each cell fraction, compared to 40% RH (p