Inactivation of microorganisms by photocatalytic nanostructures under dry conditions

The endospores of certain spore-forming bacteria are very resistant to harsh environments and thus difficult to eliminate from foods or the local environment. A need therefore exists for development of improved methods for rapid inactivation of bacterial endospores. Many studies have addressed inactivation of spores in aqueous systems, but very limited information is available on photocatalytic inactivation of endospores on surfaces in air. Methods to study photocatalytic inactivation of endospores on surfaces in air were therefore developed in the present research. These methods included spore purification, particle distribution on surfaces and recovery and enumeration of viable spores from surfaces.;Five methods for bacterial endospore purification (ASTM E2111-00, ethanol, heat shock, daily water washes, and lysozyme treatment) were applied to suspensions of Bacillus cereus. Ethanol treatment was optimum for purifying 3-day cultures, whereas the ASTM method was most favorable for 10-day cultures.;Distribution quality of spores and mixture of spore and photocatalyst on surfaces was quantitatively and qualitatively evaluated. Spores and photocatalyst distributed on Anodisc membranes by filtration method demonstrated the most uniform distributions of spores and mixture on surfaces, whereas the distribution quality of particles over plastic by a pipetting method was the worst. The distribution quality of spores and photocatalyst on glass by a pipetting method was inferior to Anodisc membranes but superior to plastic.;Enumeration of viable spores on plastic, glass and membrane surfaces by different surface sampling methods (lifting of spores from glass and plastic by polyvinyl alcohol (PVA) or removal of spores from Anodisc membranes by sonication) was compared and investigated. A 100% recovery percentage and a high sampling consistency were achieved by sonication in conjunction with Anodisc membranes. Recovery of viable spores from glass by the PVA method gave the lowest recoveries. Use of PVA to lift spores from plastic gave good recoveries.;Enhanced inactivation rate by TiO2 has been widely investigated. However, few experiments have been carried out with bacterial spores, even less for dry state inactivation. A controversy exists that contacting with titania didn't improve disinfection rate under UVA. In this study, it was found that contact of UVA irradiated spores with titania did not give higher inactivation rate coefficients than UVA irradiation of spores in the absence of titania at high light intensity. However, at light intensities of 10 to 30 W/m2, significantly higher inactivation rate coefficients were achieved in the presence of titania. Thus, light intensity plays an important role in the difference of inactivation rate under UVA irradiation in the presence and absence of titania.