Bacterial pathogens such as Staphylococcus aureus and Staphylococcus epidermidis can survive in different types of cells including professional phagocytes, causing intracellular
infections.
Antibiotic treatment of intracellular
infections is often unsuccessful due to the low efficacy of most
antibiotics inside cells. Therefore, novel techniques which can improve intracellular activity of
antibiotics are urgently needed. We aimed to use photochemical internalization (PCI) to enhance cytosolic release of
antibiotics from endocytic vesicles after internalization. Our results show that PCI indeed caused cytosolic release of
gentamicin and significantly increased its efficacy against S. epidermidis in vitro in mouse macrophages. Upon illumination for 15 min, the killing of intracellular S. epidermidis in RAW 264.7 cells by 10 or 30 μg/ml
gentamicin was increased to 1 or 3 CFU log, respectively, owing to the use of PCI, whereas no killing by
gentamicin only without PCI was observed. Moreover, survival of S. aureus-infected zebrafish embryos was significantly improved by treatment with PCI-
gentamicin. PCI improved the therapeutic efficacy of
gentamicin at a dose of 0.1 ng per embryo to a level similar to that of a dose of 0.4 ng per embryo, indicating that PCI can lower the
antibiotic dose required for treating (intracellular)
staphylococcal infection. Thus, the present study shows that PCI is a promising novel approach to enhance the intracellular efficacy of
antibiotics via cytosolic release, allowing them to reach intracellular bacteria. This will expand their therapeutic window and will increase the numbers of
antibiotics which can be used for treatment of intracellular
infections.