STX1A and infection: The first method involves direct damage to the mucosa and epithelial barrier, as infection of polarized T84 cells with O157:H7 STEC strain CL56 (Stx1a+Stx2a+) caused increases in permeability for Stx, as noted by decreased transepithelial electrical resistance (TEER) values and disruption of zonula occludin (ZO-1) staining [18].