Germicidal UV sensitivity of bacteria in aerosols and on contaminated surfaces

We compared the UV sensitivity of Bacillus atrophaeus (a surrogate for B. anthracis), Pantoea agglomerans (a bacterial simulant frequently used in biodefense studies), and Yersinia ruckeri (a surrogate for Yersinia pestis) either airborne or deposited on a semisolid (wet) agar surface. Bacterial vegetative cells were aerosolized into an exposure chamber and exposed for various lengths of time to an ultraviolet (UV) light source emitting at 254 nanometer (nm) (in the UVC region also known as UVGI). Aerosols were collected onto gelatin filters, which were dissolved, diluted, plated, and incubated to enumerate colony formation. In darkness (with the UV light switched off), it took between 170 and 330 s airborne (depending on the bacterial species) to decrease by 90% of the original load (i.e., by 1 Log10) the number of viable organisms originally present. The fluence of UVC required to inactivate 90% (F-1 Log10) of bacteria aerosolized into an atmosphere with 80-90% relative humidity corresponded to 70.3 J.m^-2, 73.3 J.m^-2 and 18.3 J.m ^-2 for vegetative cells of B. atrophaeus, P. agglomerans, and Y. ruckeri cells, respectively. Additionally, the UV sensitivity of bacteria deposited directly on agar nutrient plates was determined after exposure to 254-nm UV with the F_-1Log10 for the same bacteria on surfaces corresponding to 128 J.m^-2, 28.1 J.m^-2, and 16.3 J.m ^-2, respectively. These comparative results among different bacterial species, either airborne or on contaminated surfaces, should assist in predicting the survival of bacterial cells after transmission from infected patients or after an intentional release into the environment.