Quantum dots as enhancers of the efficacy of bacterial lethal photosensitization

Abstract

Because of the increasing resistance of bacteria to antibiotics there is considerable interestin light-activated antimicrobial agents (LAAAs) as alternatives to antibiotics fortreating localized infections. The purpose of this study was to determine whetherCdSe/ZnS quantum dots (QD) could enhance the antibacterial activity of theLAAA, toluidine blue O (TBO). Suspensions of Staphylococcus aureus andStreptococcus pyogenes were exposed to white light (3600 lux) and TBO (absorbancemaximum = 630 nm) in the presence and absence of 25 nm diameter QD (emissionmaximum = 627 nm). When the TBO:QD ratio was 2667:1, killing of Staph. aureus was enhanced by1.72log10 units. In the case of Strep. pyogenes, an enhanced kill of1.55log10 units was achieved using TBO and QD in the ratio 267:1. Singlet oxygen and fluorescencemeasurements showed that QD suppress the formation of singlet oxygen from TBO andthat QD fluorescence is significantly quenched in the presence of TBO (70–90%). Enhancedkilling appears to be attributable to a non-Förster 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 bactericidalradicals. QD may, therefore, be useful in improving the efficacy of antimicrobialphotodynamic therapy.