Lethal photosensitisation of bacteria using Silica-TBO nanoconjugates

Pathogenic microorganisms are gradually becoming resistant to antibiotics, thereby novel antimicrobial technologies are urgently needed. Photodynamic therapy (PDT) is a process that employs the energy of photons to generate reactive oxygen species through a class a chemicals known as photosensitisers. PDT has shown antimicrobial activity as the oxygen reactive species can inactivate microorganisms, at the same time, the doses required to provide antimicrobial actions are not lethal to mammalian cells. We covalently bound Toluidine blue O (TBO), a very common and safe photosensitiser, to silica nanoparticles. The conjugates exhibited antimicrobial activity against MRSA, S. epidermidis and E. coli when irradiated with laser light at 630 nm. Using a light source with a power of 500 mW the bacterial reduction exhibited a dose-response behaviour and it was 2 log10 for S. epidermidis and E. coli after 2 and 3 min, respectively. No antimicrobial activity was exhibited by the unconjugated nanoparticles or by the laser light alone. The release of TBO from the nanoparticles was pH dependent with higher amounts of photosensitisers detected at pH = 4 than pH = 7 consistent with the formation of amide bonds between nanoparticles and TBO. The light activated nanoparticles developed in this work offer a platform for the controlled delivery of TBO through a pH responsive mechanism for antimicrobial applications