Although several details of the photosensitization mechanisms involved in the photosensitized inactivation of bacteria have been elucidated, there are relatively few data on the morphological alterations induced on the bacterial cell structure during photosensitization. In this work we analysed the photodynamic action of the tetra-cationic photosensitizer tetrakis(N-ethylpyridinium-4-yl)porphyrin tetratosylate (TN-Et-PyP) on the integrity and selected functions of E. coli KMY1 cell membranes, in an effort to combine electron microscopy data with enzymatic assays and electrochemistry measurements. Using low concentrations of photosensitizer, damage is inflicted to the outer membrane and results in a higher permeability of the membrane to fairly small molecules such as deoxycholate; however, larger molecules such as periplasmic alkaline phosphatase are not released or are released after their extensive inactivation, as we could not register any enzyme activity outside the cells. Increasing the TN-Et-PyP concentration correlates with the inactivation of the respiratory chain, drop in plasma membrane voltage, the release of compounds with absorption band at 260 nm, and a decrease in intracellular enzyme β-galactosidase activity, though this activity has not been noticed to increase outside the cells, suggesting that enzyme inactivation probably occurs in inner cell districts.