Charge effect on the photoinactivation of Gram-negative and Gram-positive bacteria by cationic meso-substituted porphyrins

Abstract

BackgroundIn recent times photodynamic antimicrobial therapy has been used to efficiently destroy Gram (+) and Gram (-) bacteria using cationic porphyrins as photosensitizers. There is an increasing interest in this approach, namely in the search of photosensitizers with adequate structural features for an efficient photoinactivation process. In this study we propose to compare the efficiency of seven cationic porphyrins differing in meso-substituent groups, charge number and charge distribution, on the photodynamic inactivation of a Gram (+) bacterium (Enterococcus faecalis) and of a Gram (-) bacterium (Escherichia coli). The present study complements our previous work on the search for photosensitizers that might be considered good candidates for the photoinactivation of a large spectrum of environmental microorganisms.ResultsBacterial suspension (107 CFU mL-1) treated with different photosensitizers concentrations (0.5, 1.0 and 5.0 μM) were exposed to white light (40 W m-2) for a total light dose of 64.8 J cm-2. The most effective photosensitizers against both bacterial strains were the Tri-Py+-Me-PF and Tri-Py+-Me-CO2Me at 5.0 μM with a light fluence of 64.8 J cm-2, leading to > 7.0 log (> 99,999%) of photoinactivation. The tetracationic porphyrin also proved to be a good photosensitizer against both bacterial strains. Both di-cationic and the monocationic porphyrins were the least effective ones.ConclusionThe number of positive charges, the charge distribution in the porphyrins' structure and the meso-substituent groups seem to have different effects on the photoinactivation of both bacteria. As the Tri-Py+-Me-PF porphyrin provides the highest log reduction using lower light doses, this photosensitizer can efficiently photoinactivate a large spectrum of environmental bacteria. The complete inactivation of both bacterial strains with low light fluence (40 W m-2) means that the photodynamic approach can be applied to wastewater treatment under natural light conditions which makes this technology cheap and feasible in terms of the light source.

In vitro antimicrobial activity of light-activated phthalocyanines

“…Our results confirmed that the molecular structure [20] and electric charge [10,21] have crucial influence on the effectiveness of the photosensitive substances tested. A considerable difference in the sensitivity of Gram-positive and Gram-negative bacteria and yeast was noted [22].…”

“…During photodynamic inactivation of Gram-negative cells, PS with polycationic molecular structures -created naturally or artificially by binding the positively charged chain -have been used. These substances are therefore better able to bind and penetrate through the negatively charged lipopolysaccharide into Gram-negative bacterial cells [10][11][12].…”

Dirty hands: photodynamic killing of human pathogens like EHEC, MRSA and Candida within seconds

“…[14][15][16][17][18][19] Photodynamic inactivation of microorganisms is based on the concept of positively charged photosensitizers that can attach and/or accumulate in or at the pathogen to induce irreversible damage upon light activation of the photosensitizer. 20,21 The absorption of light by a photosensitizer leads to the generation of reactive oxygen species such as singlet oxygen which induces irreversible oxidative damage of the pathogens during illumination. 22 The presence of multiple positive charges enables the photosensitizer agent to interact with the negatively charged outer cell wall areas of bacteria, in particular with the negatively charged lipopolysaccarides of Gramnegative bacteria.…”

Non-conventional therapeutics for oral infections

“…Despite the general truth of this, recent data have shown that spontaneous mutants that are resistant to photoactivated killing can arise. 27 A sensitizer should bind avidly to the bacterial cell and/or be taken up by it, thus a number of sensitisers are highly charged molecules, 28 the most commonly tested of which have been tricyclic dyes, (e.g., methylene blue, erythrosine), phenothiazine dyes (e.g. toluidine blue O), tetrapyrroles (e.g., porphyrins) and furocoumarins (e.g.…”