LIGHT

Antibiotic resistant bacteria are a therapeutic challenge to which new solutions must be found. Staphylococcus aureus is a human pathogen that causes local and systemic infections which can be life threatening. Methicillin resistant S.aureus (MRSA) is well known as a principal cause of nosocomial infection characterised by broad spectrum resistance to most antibiotics. Recalcitrance to antibiotic therapy in S.aureusis also associated with a certain sub-population of cells known as small-colony variants (SCVs). SCVs are known to reside intracellularly in a quiescent state where they evade host defences and antimicrobial therapy, and are a reservoir for recurrent infections. SCVs are typically defective in electron transport, most commonly displaying mutations in the operons encoding menaquinone or heme biosynthesis. S.aureus is a standard laboratory strain from which two SCV mutants have been generated. Strains I10 and DB24 are insertionally inactivated in the hemB and menD genes, respectively, yielding hemin and menadione auxotrophs.

Photodynamic therapy (PDT) is a potential alternative to antibiotic therapy. PDT utilises the application of non-toxic photosensitiser dyes in combination with light of a specific wavelength resulting in the generation of reactive oxygen species which are lethal to bacterial cells. PDT is especially suited to the killing of bacteria in chronically infected, non-healing dermal ulcers.

Here we have shown that PDT using toluidine blue (TBO) is effective against both MRSA and genetically modified SCV strains, with kills of up to 6 logs observed. We have also demonstrated effective delivery and activation of TBO from a pre-loaded hydrogel.