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Nucleic acid changes during photodynamic inactivation of bacteria by cationic porphyrins

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This study, conducted by a team of Portuguese researchers, shows that photodynamic inactivation (PDI), an alternative to the use of antibiotics or as a co-adjuvant therapy for the treatment of superficial and localized skin infections and of the oral cavity, do not affect directly Gram-negative and Gram-positive bacteria nucleic acids. PDI relies on the application of a photosensitizer, in this case cationic porphyrins that after irradiation with visible light in the presence of oxygen are able to produce reactive oxygen species (ROS) that are highly cytotoxic and can inactivate cells. The results of this research, carried out in the Departments of Biology (CESAM) and Chemistry (QOPNA) of the University of Aveiro demonstrated that bacterial DNA and RNA are only affected by PDI when the bacteria are already inactivated or non-viable, and thus, the possibility that bacteria can develop resistance mechanisms to this kind of treatment is very low or even absent. While antibiotics act on a specific cellular constituent, such as a key fitting into a lock, PDI by the action of ROS, acts upon various components such as proteins, lipids, enzymes. One of the main advantages of PDI over common antibiotics is precisely the fact that it is considered a multi-target therapeutic approach, to which “photoresistance” has not yet been evidenced.

Nucleic acid changes during photodynamic inactivation of bacteria by cationic porphyrins. Global Medical Discovery











Journal Reference

Alves E1, Faustino MA2, Tomé JP2, Neves MG2, Tomé AC2, Cavaleiro JA2, Cunha A1, Gomes NC1, Almeida A1. Bioorg Med Chem. 2013 ;21(14):4311-8.

1Department of Biology & CESAM, University of Aveiro, 3810-193 Aveiro, Portugal.

2Department of Chemistry & QOPNA, University of Aveiro, 3810-193 Aveiro, Portugal.


 Light activation of photosensitizing dyes in presence of molecular oxygen generates highly cytotoxic reactive oxygen species leading to cell inactivation. Nucleic acids are molecular targets of this photodynamic action but not considered the main cause of cell death. The in vivo effect of the photodynamic process on the intracellular nucleic acid content of Escherichia coli and Staphylococcus warneri was evaluated herein. Two cationic porphyrins (Tetra-Py(+)-Me and Tri-Py(+)-Me-PF) were used to photo inactivate E. coli (5.0uM; 10(8)cells mL(-1)) and S. warneri (0.5uM; 10(8)cells mL(-1)) upon white light irradiation at 4.0mWcm(-2) for 270min and 40min, respectively. Total nucleic acids were extracted from photosensitized bacteria after different times of irradiation and analyzed by agarose gel electrophoresis. The double-stranded DNA was quantified by fluorimetry and the porphyrin binding to bacteria was determined by spectrofluorimetry. E. coli was completely photoinactivated with both porphyrins(5.0uM), whereas S. warneri was only completely inactivated by Tri-Py(+)-Me-PF (0.5uM). The hierarchy of nucleic acid changes in E. coli was in the order: 23S rRNA>16S rRNA>genomic DNA. The nucleic acids of S. warneri were extensively reduced after 5min with Tri-Py(+)-Me-PF but almost unchanged with Tetra-Py(+)-Me after 40min of irradiation. The amount of Tri-Py(+)-Me-PF bound to E. coli after washing the cells is higher than Tetra-Py(+)-Me and the opposite was observed for S. warneri. The binding capacity of the photosensitizers is not directly related to the PDI efficiency ornucleic acid reduction and this reduction occurs in parallel with the decrease of surviving cells.

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