Biofilm formation by non-O157 STEC on a polystyrene surface at 22 °C (A) and 37 °C (B) using M9 medium. The vertical axis represents the median OD 590 nm of eight replicates of each strain. Horizontal lines represent the cut-off values between weak, intermediate and strong biofilm producers. The OD is defined as three standard deviations above the mean OD of the negative control. Strains were classified as follows: OD ≤ ODc (0.082), non-adherent; ODc < OD ≤ 2 X ODc, weakly adherent; 2 X ODc < OD ≤ 4 X ODc, moderately adherent; 4 X ODc < OD strongly adherent. OD, optical density; ODc, cut-off OD value.