Enhanced environmental stress tolerance in reoccurring, emerging, and persistent (REP) Escherichia coli O157:H7 strains from leafy green associated outbreaks.

Escherichia coli O157:H7, historically associated with undercooked beef, has increasingly been linked to leafy green associated outbreaks in the United States. A subset of strains, termed reoccurring, emerging, and persistent (REP), represented by REPEXH01 and REPEXH02, has repeatedly caused multistate outbreaks since 2017, raising concerns about their enhanced environmental fitness and persistence in leafy green production regions. In this study, we evaluated the stress tolerance profiles of representative REPEXH01 (PNUSAE013458) and REPEXH02 (2705C and PNUSAE019890) strains, using the meat-associated strain EDL933 and the spinach outbreak strain EC4115 as references. REPEXH01 strains exhibited elevated tolerance to starvation, arsenic, gastric acid, and antibiotics, traits consistent with genomic islands encoding multiple resistance determinants. REPEXH02 strains showed increased tolerance to UV-C irradiation, potentially linked to mutations in DNA repair genes. Both REP groups showed significantly higher survival than EDL933 under desiccation and blue light exposure (major antimicrobial spectrum in sunlight), underscoring their enhanced persistence in agricultural and processing environments. These findings highlight distinct adaptation strategies between REPEXH01 and REPEXH02 that may facilitate their continued recurrence in leafy green outbreaks. Understanding these stress response mechanisms provides critical insights into REP strain ecology and informs the development of improved intervention strategies to reduce produce-associated E. coli O157:H7 contamination.