Pristine titanium dioxide (TiO 2 ) absorbs ultraviolet light and reflects the entire visible spectrum. This optical response of TiO 2 has found widespread application as white pigments in paper, paints, pharmaceuticals, foods and plastic industries; and as a UV absorber in cosmetics and photocatalysis. However, pristine TiO 2 is considered to be inert under visible light for these applications. Here we show for the first time that a bacterial contaminant (Staphylococcus aureus-a MRSA surrogate) in contact with TiO 2 activates its own photocatalytic degradation under visible light. The present study delineates the critical role of visible light absorption by contaminants and electronic interactions with anatase in photocatalytic degradation using two azo dyes (Mordant Orange and Procion Red) that are highly stable because of their aromaticity. An auxiliary light harvester, polyhydroxy fullerenes, was successfully used to accelerate photocatalytic degradation of contaminants. We designed a contaminant-activated, transparent, photocatalytic coating for common indoor surfaces and conducted a 12-month study that proved the efficacy of the coating in killing bacteria and holding bacterial concentrations generally below the benign threshold. Data collected in parallel with this study showed a substantial reduction in the incidence of infections.Patients and visitors in healthcare facilities can acquire infections by direct or indirect contact with common surfaces (room door handles, bed rails, taps, sterile packaging, mops, ward fabrics and plastics, keyboards and telephones) that have become contaminated with pathogenic microbes 1 . Making these surfaces microbe-unfriendly can break the cycle of contamination and infection. Antimicrobial coatings that slowly release toxic silver or copper ions, currently in clinical trials [2][3][4][5] , have limited lifetime, are difficult to apply and are costly [6][7][8][9] . Further, copper surfaces were unable to reduce bacterial concentrations to the benign level in clinical trials 2,4 . TiO 2 photocatalysis has attracted intense interest for applications in self-cleaning and antimicrobial coatings as TiO 2 can completely mineralize organic contaminants including microorganisms and the process produces no toxic by-products [10][11][12] . Further, TiO 2 is environmentally benign and inexpensive 13,14 . Unfortunately, TiO 2 , which is an excellent photocatalyst under UV light, has very limited capability for visible light absorption 11,15,16 . Extension of TiO 2 photocatalysis to visible light is a highly active area of research [17][18][19][20][21][22][23][24] . Major approaches are: 1) creation of defects within the TiO 2 crystalline structure, such as oxygen or titanium vacancies or substitutions. Techniques include doping (with elements such as, carbon, nitrogen, sulfur or phosphorous), annealing in reducing atmospheres and synthesis in the presence of reductants [24][25][26][27] ; 2) creation of defects at the TiO 2 surface. Techniques include surface hydrogenation, plasma tr...
