PBFI-AM staining

Potassium efflux was confirmed by investigating the intracellular K⁺ content of the cells using the cell permeable, fluorophore PBFI-AM (Abcam, Cambridge, United Kingdom). After 48 h infection or treatment with the porphyrin extract, supernatant was removed and the cells were washed with Hanks balanced salt solution (HBSS; Gibco). 400 μL loading buffer (PowerLoad, Invitrogen) was added to the wells and 10 μM PBFI-AM and 10 μM Pluronic F-127 (Invitrogen) were included per well, after which the plate was incubated, protected from light, for 2h at 37°C. Afterwards, the loading buffer was removed and the cells were washed with HBSS. Imaging was accomplished using the EVOS FL Auto Imaging System equipped with a 10x objective (excitation: 357 nm, emission: 447 nm, final magnification: 300x). Fluorescence intensity was measured using the ImageJ software.