Siderophore-
photosensitizer conjugates, their synthesis and use in photodynamic 
antimicrobial therapy (PACT) is disclosed. The 
advantage of this method is improvement of photodynamic 
antimicrobial therapy against, for example, pathogenic micro-organisms such as 
bacteria and fungi. Naturally occurring and synthetically available 
siderophore structures are conjugated chemically with photoactive compounds such as 
Chlorin e6 to improve their penetration into bacterial cells and to increase 
antibacterial efficacy of photosensitizers via microbial proteins that recognize and transport iron-loaded siderophores. In this way, photosensitizers can be transported inside 
bacteria that otherwise could not cross the 
cell wall and membranes. Photodynamic activation of photosensitizers inside the cells of pathogenic microbes enables a more effective inhibition of 
cellular functions than application at the outer side of the cells. The 
siderophore-transporting systems of microbes are known to be specific for 
bacteria and fungi. Consequently, 
siderophore conjugates with photosensitizers are not taken up by mammalian cells and photodynamic effects can thus be exerted specifically on pathogenic microbes. Applications of these conjugates include highly efficient treatment of pathogenic 
gram-negative and -positive bacteria such as 
Pseudomonas aeruginosa, 
Escherichia coli, 
Streptococcus pyogenes, 
Staphylococcus aureus, treatment of microbial infections that often occur in chronic wounds as well as therapy of other antibiotic resistant microbial infections.