Curli fimbriae are conditionally required in Escherichia coli O157:H7 for initial attachment and biofilm formation

• Curli enhance the attachment of Escherichia coli O157:H7 to plant and abiotic surfaces.
• Curli are required for E. coli O157:H7 biofilm formation on steel surface.
• Curli increase E. coli O157:H7 biofilm production on glass surface.
• Curli contribute minimally to biofilm formation in cells grown in spinach lysates.
• Curli promote mixed biofilm by E. coli O157:H7 and plant-associated microorganisms.

Several species of enteric pathogens produce curli fimbriae, which may affect their interaction with surfaces and other microbes in nonhost environments. Here we used two Escherichia coli O157:H7 outbreak strains with distinct genotypes to understand the role of curli in surface attachment and biofilm formation in several systems relevant to fresh produce production and processing. Curli significantly enhanced the initial attachment of E. coli O157:H7 to spinach leaves and stainless steel surfaces by 5-fold. Curli was also required for E. coli O157:H7 biofilm formation on stainless steel and enhanced biofilm production on glass by 19–27 fold in LB no-salt broth. However, this contribution was not observed when cells were grown in sterile spinach lysates. Furthermore, both strains of E. coli O157:H7 produced minimal biofilms on polypropylene in LB no-salt broth but considerable amounts in spinach lysates. Under the latter conditions, curli appeared to slightly increase biofilm production. Importantly, curli played an essential role in the formation of mixed biofilm by E. coli O157:H7 and plant-associated microorganisms in spinach leaf washes, as revealed by confocal microscopy. Little or no E. coli O157:H7 biofilms were detected at 4 °C, supporting the importance of temperature control in postharvest and produce processing environments.