Quantum dots as enhancers of the efficacy of bacterial lethal photosensitization

Because of the increasing resistance of bacteria to antibiotics there is considerable interest in light-activated antimicrobial agents (LAAAs) as alternatives to antibiotics for treating localized infections. The purpose of this study was to determine whether CdSe/ZnS quantum dots (QD) could enhance the antibacterial activity of the LAAA, toluidine blue O (TBO). Suspensions of Staphylococcus aureus and Streptococcus pyogenes were exposed to white light (3600 lux) and TBO (absorbance maximum = 630 nm) in the presence and absence of 25 nm diameter QD (emission maximum = 627 nm). When the TBO: QD ratio was 2667: 1, killing of Staph. aureus was enhanced by 1.72 log(10) units. In the case of Strep. pyogenes, an enhanced kill of 1.55 log(10) units was achieved using TBO and QD in the ratio 267: 1. Singlet oxygen and fluorescence measurements showed that QD suppress the formation of singlet oxygen from TBO and that QD fluorescence is significantly quenched in the presence of TBO (70 - 90%). Enhanced killing appears to be attributable to a non-Forster resonance energy transfer mechanism, whereby the QD converts part of the incident light to the absorption maximum for TBO; hence more light energy is harvested, resulting in increased concentrations of bactericidal radicals. QD may, therefore, be useful in improving the efficacy of antimicrobial photodynamic therapy.