Ultraviolet disinfection efficacy test method using bacteria monolayers

Monolayers of bacterial cells of Staphylococcus aureus and Pseudomonas aeruginosa were inoculated on glass slide carriers using an automated inoculum spray deposition system. The use of bacterial monolayers allows for control of critical variables for testing and verification of light-based disinfection technologies. This approach avoids the variability associated with manual inoculation and high inoculum titers, which can engender clustering of cells and the associated photoprotection that clustering incurs. The use of glass slide carriers avoids problems caused by irregular microscopic surface features, which can impact the efficacy evaluation of light-based disinfection technologies. Scanning electron micrographic (SEM) imaging was used to verify the surface topography and the presence of monolayers. The spray deposition method produced a mean density of >10⁶ colony forming units (CFU) per carrier. The inoculated carriers were exposed to ultraviolet light for 120 s from a focused multivector ultraviolet (FMUV) light system. A mean log CFU reduction of 4.8 was achieved for S. aureus (p < 0.0001). A mean log CFU reduction of 5.1 was achieved for P. aeruginosa (p < 0.0001). The test method presented herein will facilitate increased accuracy in the measurement of ultraviolet susceptibility rate constants.