Biofilm formation of Escherichia coli O157:H7 strains associated with recent reoccurring lettuce outbreaks

Authors: YANG, YISHAN - Oak Ridge Institute For Science And Education (ORISE); Yan, Xianghe; REDDING, MARINA - Oak Ridge Institute For Science And Education (ORISE); Gu, Ganyu; Luo, Yaguang; Patel, Jitendra; Nou, Xiangwu

Submitted to: Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/8/2025
Publication Date: 1/10/2025
Citation: Yang, Y., Yan, X., Redding, M., Gu, G., Luo, Y., Patel, J.R., Nou, X. 2025. Biofilm formation of Escherichia coli O157:H7 strains associated with recent reoccurring lettuce outbreaks. Food Control. https://doi.org/10.1016/j.fm.2025.104728.
DOI: https://doi.org/10.1016/j.fm.2025.104728

Interpretive Summary: In recent years, several E. coli O157:H7 outbreaks have been linked to romaine lettuce. The CDC and FDA have designated these outbreak-associated E. coli O157:H7 strains as "reoccurring, emerging, and persistent (REP)". In this study, ARS scientists demonstrated that the E. coli O157:H7 REP strains had much stronger ability to form biofilms and resist common sanitizers compared to other strains linked to previous outbreaks involving lettuce, spinach, and hamburgers. These REP strains also demonstrated remarkable tolerance to stomach acid. The enhanced ability to form biofilms appears to be due to their increased production of curli and cellulose. A better understanding of how these REP strains persist offers valuable insights that could lead to improve strategies for preventing future outbreaks, ultimately helping to protect public health.

Technical Abstract: Clustered genomically close E. coli O157:H7 strains have been implicated in several recent multistate outbreaks linked to romaine lettuce. The underlying factors contributing to their reoccurrence and persistence remain elusive. Biofilm formation and acid resistance are crucial factors for foodborne pathogens in their environmental persistence and success in host gastrointestinal invasion. Thus, the objective of this study was to investigate the biofilm-forming capability of outbreak strains, their resistance to antimicrobials, and their tolerance to gastric acid, by comparing O157:H7 strains associated with recent reoccurring outbreaks and those associated with previous lettuce, spinach, and hamburger outbreaks. The recent outbreak strains, which were collectively described as “reoccurring, emerging, and persistent (REP)”, exhibited significantly stronger biofilm-forming capabilities and resistance to quaternary ammonium compounds (QACs) compared to other strains. They also exhibited strong tolerance to simulated gastric fluid. Their ability to form robust biofilms is likely attributed to their pronounced production of curli and cellulose, as demonstrated on Congo Red and Calcoflour White agar plates. Moreover, their exceptional resistance to sanitizers may stem from the formation of dense biofilms with higher cellulose content, as visualized using fluorescent dyes under confocal laser scanning microscopy. The findings of this study support the assertion that biofilm formation is a critical factor for the reoccurring outbreak strains for environmental persistence and provide insights for developing prevention strategies.