Method of enhancing the antimicrobial action of systemically administered antibiotics

Antibiotic for use in a method comprising systemically administering a photosensitive antibiotic to a subject having a microbial infection in a target tissue to provide a blood plasma concentration of the antibiotic in the subject; allowing the plasma or tissue concentration of the antibiotic increase to a predetermined level; and subjecting the target tissue to photodynamic therapy in order to increase the antimicrobial action of the antibiotic in the target tissue. With light activation the efficacy of the photosensitizer antibiotic can be enhanced, the spectrum of the microbial effect broadened and even an antiviral effect can be obtained. Generally, the present invention can be exploited when there is a need for enhancement of the local antimicrobial effect of antibiotics.