Erythrosine-Based PDT of Oral Epithelial Cells Figure 2. Cell Killing Assay. The pre-malignant (DOK) and malignant (H357) oral epithelial cells were incubated with different concentrations of erythrosine B (71.03 mM, 142.06 mM, 284.13 mM, 568.27 mM and 1136.50 mM) followed by PDT irradiation with different fluences (0 J/cm2, 40.86 J/ cm2, 81.72 J/cm2 and 122.58 J/cm2). Following PDT, the cells were recovered 24 h later and percentage (%) of cell killing caused by PDT was estimated. (A) DOK cell’s susceptibility to erythrosine-based PDT and (B) H357 cell’s susceptibility to erythrosine-based PDT. The values are means of six replicate determinations 6 S.D. Statistical analysis was done via student’s t-test with significance set at P,0.05 (* indicates statistical comparison of a data-point within one erythrosine B concentration group vs. the corresponding data values of 0 J/cm2 fluencies for that group). doi:10.1371/journal.pone.0034475.g002 Erythrosine-PDT affects Mitochondrial Trans-membrane Potential (DYm) in DOK and H357 cells DOK and H357 cells treated with PDT using a low dose of erythrosine-PDT (71.03 mM erythrosine concentration and 122.58 J/cm2 irradiation fluence) showed different responses to the treatment in terms of changes in the DYm. Following low dose treatment, DOK cells demonstrated a considerable decrease in their overall DYm immediately following PDT (Figure 4A). Interestingly, at 2 hrs recovery time, a specific population of cells PLoS ONE | www.plosone.org 4 April 2012 | Volume 7 | Issue 4 | e34475