Biofilm Formation of Non-O157 Shiga Toxin-producing Escherichia coli (STEC) on Equipment Surfaces

Introduction: Shiga toxin-producing Escherichia coli (STEC) serotype O157:H7 has been the most commonly recognized STEC serotype responsible for foodborne outbreaks in the US.  Numerous outbreaks associated with non-O157 serotypes have also been reported due to consumption of contaminated food. The six major serotypes, referred to as "the big six," can attach and form biofilms on food contact surfaces, and subsequently contaminate food at the processing facility. 

Purpose: The purpose of this study was to compare different non-O157 STEC for curli expression and biofilm formation.

Methods: Biofilm formation of 12 non-O157 serotypes (O26, O45, O103, O111, O121, O145) was determined using a CDC Biofilm Reactor assay. Stainless steel, PTFE and polycarbonate coupons (n=180) were used in the reactor containing 400 ml of 10% nutrient broth (NB) which had been inoculated with 1 ml of overnight culture. The reactor was set with a continuous flow of 10% NB and shear forces of a swirling paddle for 24 h. After 24 h, coupons were scraped off to remove attached non-O157 and populations of non-O157 were determined by plating procedure. Curli expression of strains was identified by their affinity to Congo red (CR) dye.

Results: Biofilm formation of non-O157 serotypes varied with material surface and strain. Most strains were recovered at significantly higher levels on PTFE and polycarbonate surfaces compared to populations recovered on stainless steel surfaces. E. coli O26 strain 3629 recovered from stainless steel, polycarbonate and PTFE (7.06 to 7.44, log CFU/cm²) were significantly higher than E. coli O145 strain 3419 recovered from corresponding surfaces. In general, curli-expressing strains formed stronger biofilms on material surfaces.

Significance: Biofilm formation of non-O157 serotypes varies among EHEC serotypes and is influenced by the equipment surface. Understanding biofilm mechanisms will be helpful in evaluation of contamination risk and intervention strategies to prevent contamination of foods.