Photodynamic treatment by the cationic TMPyP photosensitizer was undertaken on the multiple antibiotic-resistant bacteria Acinetobacter baumannii and Escherichia coli. Total eradication of the bacterial cultures was determined immediately after initiation of illumination when these bacteria were treated with 5, 10, 15, 20-tetra (4-N methylpyridyl)porphine (TMPyP) at a concentration of 29.4 IJ mom and illuminated by blue, green, or red light. Total eradication of both bacteria was obtained also after treatment of bacterial cultures with 3.7 mu mol/L TMPyP and illumination with blue light (400-450 nm). On the other hand, an 8- or 16- to 20-fold higher light intensity, respectively, was required for total eradication upon illumination with green (480-550 nm) or red light (600-700 nm). A 407-nm blue light only 7 and 9 joules/cm(2), respectively, was needed for total eradication of both bacteria even at a concentration of 3.7 mu mol/L TMPyP. X-ray-linked microanalysis demonstrated loss of potassium and a hood of sodium and chloride into the cells, indicating serious damage to the cytoplasmic membrane. Transmission electron microscopy (TEM) revealed structural changes and damage to the membrane of treated E. coli. In A. baumannii-treated cells, mesosomes and black dots that resemble aggregation of polyphosphate polymers could be seen. DNA breakage appeared only after a long period of illumination, when the bacterial cell was no longer viable. It can be concluded that cytoplasmic membrane damage and not DNA breakage is the major cause for bacterial death upon photosensitization.