2.11. Measurement of Transepithelial Electrical Resistance (TEER) and Permeability across Caco-2 Monolayers

After seeding in transwell inserts, Caco-2 cells were cultured for at least 16 days to form monolayers. For TEER measurement, the monolayers were incubated with 1 × 10⁸ CFU of YL20 and 1 × 10⁷ CFU of C. sakazakii, followed by measuring TEER at 0, 6, 18 and 24 h after bacterial exposure using a Millicell electrical resistance apparatus (World Precision Instruments Inc., Sarasota, FL, USA). For paracellular permeability assays, the chambers were equilibrated with Hanks, followed by adding FITC-dextran (1 mg/mL) (Sigma, St Louis, MO, USA) to the upper chamber and incubation at 37 °C for 1.5 h in the dark. Next, the solutions in the lower chamber were collected, followed by measuring the fluorescence intensity at 520 nM with a microplate reader (Tecan, Zürich, Switzerland). As previously reported [23], apparent permeability coefficient (Papp) was counted by the formula: Papp = dQ/(dt × A × C₀), where dQ/dt is the permeability rate; A, the diffusion area of monolayer; and C₀, the initial concentration of the tested substance in the apical compartment.