Purpose: The purpose of this study was to evaluate the effects of natural antimicrobials in reducing cells in biofilms formed by curli-expressing E. coli O157:H7 strains.
Methods: Three morphotypes of E. coli O157:H7 86-24: wild-type, curli deficient 86-24DcsgA mutant, and curli over-expressing constitutive 86-24csgDc mutant strains were used in MBEC™ HTP assay to study biofilm formation. Following incubation in LB medium w/o salt for 48h at 25°C, biofilm formed on MBEC™ polystyrene pegs (n = 36) were analyzed by crystal violet assay. Biofilms were treated with cinnamaldehyde and Sporan at 1000-3000 µg/ml or chlorine (50 µg/ml) for 10 min to evaluate their potentials in removing biofilm. Following neutralization of antimicrobials, surviving E. coli O157:H7 populations in biofilms were dislodged by sonication and then analyzed by spiral plating on SMAC media.
Results: The biofilm formation by E. coli O157:H7 strongly correlated with their ability to produce curli. The curli over-expressing mutant produced significantly (P < 0.05) greater biomass (0.861 ± 0.046) compared to wild-type (0.296 ± 0.018) and curli-deficient strain (0.115 ± 0.010). Cinnamaldehyde and Sporan significantly reduced E. coli O157:H7 (3-5 log CFU reductions) in biofilm. The efficacy of antimicrobials in reducing cells in biofilm was increased with an increase in their concentrations. Chlorine was superior to natural antimicrobials in reducing cells in bioiflms. Cells in biofilm formed by curli over-expressing strain showed greater resistance to antimicrobials than cells formed by wild or curli-deficient strains.
Significance: Cinnamaldehyde and Sporan have potential in reducing E. coli O157:H7 in biofilm. Biofilm formed by curli over expressing E. coli O157:H7 may require higher antimicrobial concentrations to remove cells from equipment surfaces.