Mechanism of uptake of a cationic water-soluble pyridinium zinc phthalocyanine across the outer membrane of Escherichia coli

Previous studies have shown that a cationic water-soluble pyridinium zinc phthalocyanine (PPC) is a powerful photosensitizer that is able to inactivate Escherichia coli. In the current work incubation of E. coli cells with PPC in the dark caused alterations in the outer membrane permeability barrier of the cells, rendering the bacteria much more sensitive to hydrophobic compounds, with little effect seen with hydrophilic compounds. Addition of Mg21 to the medium prior to incubation of the cells with PPC prevented these alterations in the outer membrane permeability barrier. The presence of Mg21 in the medium also prevented the photoinacti-vation of E. coli cells with PPC. These results are consistent with the hypothesis that PPC gains access across the outer membrane of E. coli cells via the self-promoted uptake pathway, a mechanism of uptake postulated for the uptake of other cationic compounds across the outer membranes of gram-negative bacteria. Whereas gram-positive bacteria have generally been sensi-tive to photoinactivation with porphyrin and phthalocyanine photosensitizers (1, 3, 4, 15), gram-negative bacteria have been reported (1–4, 15, 27) to be relatively insensitive unless the photosensitizers are administered in the presence of additional substances, for example, CaCl2, Tris-EDTA, or polymyxin B nonapeptide (PMBN), that alter the permeability of the outer membrane. The insensitivity of gram-negative bacteria to pho-toinactivation was attributed to the presence of the outer mem-brane, which markedly decreases the permeability of the mem-brane for large or hydrophobic compounds compared to tha