Dose fractionation as a promising strategy to improve UV disinfection

Abstract

Survival curves for bacteria exposed to germicidal radiation (254 nm) usually exhibit a decreased slope at high doses, because bacterial populations contain a small proportion of cells able to tolerate the effects of the radiation. These cells could survive due to external shielding by aggregation or association with inert particles. However, it has been proposed that transient and reversible phenotypic changes modifying UV susceptibility also occur and lead to a phenomenon of persistence. Using Escherichia coli as a model organism, and comparing the effects of continuous exposures to those of irradiations separated by incubations in the dark, we found that in most of the survivors to high UV doses, the condition conferring tolerance (possibly self-aggregation) is transient and reversible, as would be expected in a persistent sub-population. Splitting the irradiations into two exposures reduced viable counts to the detection limit (6.32 orders of magnitude) under conditions in which continuous irradiations produced a reduction no greater than 4 orders of magnitude. In those processes where it is feasible, UV dose fractionation could improve the effectiveness of disinfection by ultraviolet radiation.

Keywords

• Ultraviolet
• Germicidal
• Disinfection
• Persistence