Photodynamic Inactivation of Bacteria in Ionic Environments Using the Photosensitizer SAPYR and the Chelator Citrate^†

Abstract

Many studies show that photodynamic inactivation (PDI) is a powerful tool for the fight against pathogenic, multiresistant bacteria and the closing of hygiene gaps. However, PDI studies have been frequently performed under standardized in vitro conditions comprising artificial laboratory settings. Under real-life conditions, however, PDI encounters substances like ions, proteins, amino acids and fatty acids, potentially hampering the efficacy of PDI to an unpredictable extent. Thus, we investigated PDI with the phenalene-1-onebased photosensitizer SAPYR against Escherichia coli and Staphylococcus aureus in the presence of calcium or magnesium ions, which are ubiquitous in potential fields of PDI applications like in tap water or on tissue surfaces. The addition of citrate should elucidate the potential as a chelator. The results indicate that PDI is clearly affected by such ubiquitous ions depending on its concentration and the type of bacteria. The application of citrate enhanced PDI, especially for Gram-negative bacteria at certain ionic concentrations (e.g. CaCl 2 or MgCl 2 : 7.5 to 75 mmol L −1 ). Citrate also improved PDI efficacy in tap water (especially for Gramnegative bacteria) and synthetic sweat solution (especially for Gram-positive bacteria). In conclusion, the use of chelating agents like citrate may facilitate the application of PDI under real-life conditions.

Photodynamic inactivation of different pathogenic bacteria on human skin using a novel photosensitizer hydrogel

“…The addition of sweat components without histidine to bacteria suspension left the PDI efficacy nearly unchanged (Table 1). The chelator citrate also here prevented the inhibitory effect of the divalent ions, whereas the impact of the other substances was negligible 45 . This is confirmed when comparing the results shown in Table 1 (with citrate) and Table S1 (without citrate).…”

“…The chelator citrate also here prevented the inhibitory effect of the divalent ions, whereas the impact of the other substances was negligible. 45 This is confirmed when comparing the results shown in Table 1 (with citrate) and Table S1 (without citrate).…”

“…Also, divalent ions like magnesium and calcium in sweat already proved to disturb PDI. They bind to bacterial cells and thereby interfere with the attachment of photosensitizers to the bacterial membranes, 34 but this can be overcome by using a chelator like citrate 45 . In summary, the next experiments particularly focused on the influence of such sweat ions and histidine as exemplary amino acid.…”

“…However, these differences in PDI efficacies were statistically not significant for S. aureus ( p = 0.050) and P. aeruginosa ( p = 0.127). Amino acids like histidine are usually very potent quenchers of singlet oxygen and may impair PDI efficacy in bacterial suspensions to a large extent 45,46 . This might be different for bacteria on surfaces.…”

“…Amino acids like histidine are usually very potent quenchers of singlet oxygen and may impair PDI efficacy in bacterial suspensions to a large extent. 45,46 This might be different for bacteria on surfaces. In bacterial suspensions, histidine and bacteria (loaded with singlet oxygen-generating SAPYR) can move freely towards each other and interact, which is not the case when dried on surfaces.…”