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

Abstract

Genomically closely clustered 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.