Photodynamic therapy for antibiotic resistant bacteria: Light-integrated gel healthcare technology (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.

Working under the project title Light-Integrated Gel Healthcare Technology (LIGHT), this research was conducted by pharmaceutical researchers working with design researchers to explore Photodynamic therapy for antibiotic resistant bacteria.

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.

The research team 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.

Antibiotic-resistant S.aureus and small-colony variants trains were found to be lethally photosensitised using a combination of red light and TBO in solution with greater than 99.9 per cent kills achieved at the most effective concentration (40μM). Using the same starting inoculum, further experiments demonstrated complete kill using 40μM TBO in combination with 30 minutes exposure to red light.

The viability of an inoculum of S.aureus NCTC 8325-4 spread on to the surface of a TBO-containing hydrogel was reduced by up to five logs when exposed to red light for 1 hour. This is suggestive of a potential for the use of a TBO-containing hydrogel as a means of treating topical S.aureus infections in combination with a red light source of appropriate intensity.

These results suggest that photodynamic therapy may be an important alternative for the treatment of bacteria infections displaying broad-spectrum antibiotic resistance. We believe that this is the first example of the use of PDT to eradicate slow growing small-colony variant strains–which are likely to become the focus of further research as their significance in nosocomial infections becomes apparent.

Lethal Photosensitisation of Staphylococcus aureus MRSA and Small-Colony Variants using a TBO-loaded Hydrogel. Bowyer, C., J. Farrer, I. Savina, I.U. Allan. Tissue Engineering and Regenerative Medicine International Society EU meeting, June 2013, Istanbul, Turkey.