Potential repair of Escherichia coli DNA following exposure to UV radiation from both medium- and low-pressure UV sources used in drinking water treatment

Abstract

The increased use of UV radiation as a drinking water treatment technology has instigated studies of the repair potential of microorganisms following treatment. This study challenged the repair potential of an optimally grown nonpathogenic laboratory strain of Escherichia coli after UV radiation from low- and medium-pressure lamps. Samples were irradiated with doses of 5, 8, and 10 mJ/cm(2) from a low-pressure lamp and 3, 5, 8, and 10 mJ/cm(2) from a medium-pressure UV lamp housed in a bench-scale collimated beam apparatus. Following irradiation, samples were incubated at 37 degrees C under photoreactivating light or in the dark. Sample aliquots were analyzed for up to 4 h following incubation using a standard plate count. Results of this study showed that E. coli underwent photorepair following exposure to the low-pressure UV source, but no repair was detectable following exposure to the medium-pressure UV source at the initial doses examined. Minimal repair was eventually observed upon medium-pressure UV lamp exposure when doses were lowered to 3 mJ/cm(2). This study clearly indicates differences in repair potential under laboratory conditions between irradiation from low-pressure and medium-pressure UV sources of the type used in water treatment.

FIG. 1.

Comparison of the relative spectral emittance (200 to 400 nm) for low- (——) and medium-pressure (----) UV lamps (reprinted from reference with permission).

FIG. 2.

Photorepair and dark repair potential of E. coli following low- and medium-pressure UV irradiation at doses of 10 (a), 8 (b), and 5 (c) mJ/cm².

FIG. 3.

Irradiation of E. coli at 10 mJ/cm² from low-pressure (a) and medium-pressure (b) UV sources showing average concentrations prior to irradiation (pre UV), following irradiation (post UV), and following incubation under photoreactivating light and dark conditions at an optimal time of 180 min. Averages and standard deviations are based on two separate experiments, each using three plates per appropriate dilution (n = 6).

FIG. 4.

Photorepair and dark repair potential of E. coli following medium-pressure UV irradiation at a dose of 3 mJ/cm².