Modeling Survival of Bacillus anthracis Spores Exposed to UVC and Simulated Solar Light

The effect of exposure to germicidal ultraviolet-C (UVC) and simulated solar light upon relative survival of Bacillus anthracis spores was measured across several experimental covariates, and corresponding statistical models were developed. These covariates included bacteria strain; Ames versus Sterne strain; UV versus simulated solar light; spore particle size; substrates including membrane filter, fiber, quartz slide, and aerosol; and the deposition method. A non-linear model, Annelis (1965),described loss of viability in response to light exposure and was supported over simpler models. The graphic model comparison method using confidence intervals was chosen for comparing covariate levels. The deposition method had an effect on variance but little effect on expected values. Confidence intervals broadly overlapped across the range of doses between most covariate levels, with the following noteworthy exceptions: small particles in aerosol and on quartz exhibited higher survival at low exposures than particles on fiber, Ames exhibited higher viability at low exposures than Sterne strain in small particles on quartz, quartz may be used as a proxy for aerosol in those conditions experimentation with aerosols is precluded, and higher survival in the pathogenic strain Ames should be accounted for when using non-pathogenic Sterne strain as a proxy in models.