Curli fimbriae confer shiga toxin-producing Escherichia coli a competitive trait in mixed biofilms

Shiga toxin-producing Escherichia coli (STEC) is one of the most common causal agents of foodborne illness linked to fresh leafy vegetables. Here, we investigated the impact of spinach-associated microorganisms on proliferation and biofilm formation of STEC O157:H7 on stainless steel surfaces at temperatures related to produce production and postharvest processing environments. Although a proliferation of inoculated pathogen cells in spinach leaf wash water was detected at all temperatures examined, the impact of spinach-associated microorganisms on the proliferation of E. coli O157:H7 was observed at 10 °C and 26 °C, but not at 4 °C. The inhibition of E. coli O157:H7 growth by spinach-associated microorganisms indicated a competition between the pathogen and spinach indigenous microflora. A significant decrease of the pathogen population in mixed biofilms was observed only at 26 °C for curli-deficient strain MQC43, but not for curli-expressing strain MQC57. Deletion of curli genes in a curli-expressing strain resulted in a phenotype similar to that of MQC43 in mixed biofilms; however, this deficiency was rescued when curli biogenesis was restored in the curli-deletion mutant strain. Our data support that curli confer E. coli O157:H7 a competitive trait in mixed biofilms, presumably through the interaction between STEC and the biofilm-proficient microorganisms associated with spinach leaves.