Introduction

The need for alternative techniques of microbial control has been determined by the rapid spread of antibiotic-resistant bacteria. One approach that is gaining popularity is antimicrobial photodynamic therapy (aPDT). This method involves the use of a chemical compound (photosensitizer), which is activated by light in the presence of molecular oxygen. The activated photosensitizer induces the production of reactive oxygen species (ROS), which leads to cell destruction. Significant attention has been directed toward the application of 5-aminolevulinic acid (5-ALA) in aPDT. This compound occurs naturally in cells, where it is converted mainly to the intracellular photosensitizer protoporphyrin IX (PpIX). Over time, PpIX is transformed into heme (which does not possess photosensitizing properties) by the ferrochelatase enzyme [1]. An increased concentration of PpIX in cells can be achieved by inhibiting ferrochelatase with divalent metal ions (e.g, Mn^{2 +,} Zn^{2 +}) [2].

Methods

The aim of this study is to evaluate the combined effect of 5-ALA (2.5 mM), light, and metal ions on the photosensitization of bacteria. The microorganism tested was antibiotic-resistant Proteus mirabilis (PCM 543). Irradiation was performed with a 404 nm diode laser (the light dose was 23.5 J cm-2). For the research, ions of manganese (1 mM), zinc (1 mM), calcium (10 mM), and magnesium (10 mM) were used.

Results

The most effective approach was found to involve the use of 5-ALA with zinc ions, resulting in the destruction of almost 99% of bacterial cells, while the use of 5-ALA alone led to the eradication of approximately 90% of cells after the same exposure time.

Conclusions

The results obtained suggest that metal ions, especially zinc, may compete with iron ions for the ferrochelatase binding site, leading to the inhibition of the enzyme and, at the same time, the accumulation of PpIX in cells.

[1] doi:10.3390/ijms25073590

[2] doi:10.1074/jbc.M803372200