Repair of DNA in bacteria following ultraviolet (UV) disinfection can cause reactivation of inactivated bacteria and negatively impact the efficiency of the UV disinfection process. In this study, various strains of E. coli (wild-type, UV-resistant and antibiotic-resistant strains) were investigated for their ability to perform dark repair and photoreactivation, and compared based on final repair levels after 4 h of incubation, as well as repair rates. Analysis of the results revealed that the repair abilities of different E. coli strains can differ quite significantly. In photoreactivation, the log repair ranged from 10 to 85%, with slightly lower log repair percentages when medium-pressure (MP) UV disinfection was employed. In dark repair, log repair ranged from 13 to 28% following low-pressure (LP) UV disinfection. E. coli strains ATCC 15597 and ATCC 11229 were found to repair the fastest and to the highest levels for photoreactivation and dark repair, respectively. These strains were also confirmed to repair to higher levels when compared to a pathogenic E. coli O157:H7 strain. Hence, these strains could possibly serve as conservative indicators for future repair studies following UV disinfection. In addition, dimer repair by photoreactivation and dark repair was also confirmed on a molecular level using the endonuclease sensitive site (ESS) assay.

Aims:  To investigate the effects of fluorescent light intensity, sunlight intensity and temperature on photoreactivation of Escherichia coli after low‐pressure (LP) and medium‐pressure (MP) ultraviolet (UV) disinfection. Methods and Results:  Two E. coli strains were irradiated with LP and MP UV lamps, and exposed to various fluorescent light (0–23 kLux) and sunlight intensities (1–80 kLux), and temperatures (4–50°C). Escherichia coli concentrations were enumerated at hourly intervals to determine photoreactivation rates and final photoreactivation levels. Higher photoreactivation rates and levels were observed with increasing fluorescent light intensities, while high sunlight intensity (>12 kLux) caused a one‐log decrease in E. coli concentrations. When exposed to near‐optimum growth temperatures (23–37°C), photoreactivation levels were higher than those with too high (50°C) or too low (4°C) temperatures. Overall, photoreactivation following MP UV disinfection was lower than that following LP UV disinfection. Conclusions:  Photoreactivation of bacteria following UV disinfection can be a problem in tropical countries where sunlight is abundant and temperatures are high, unless high sunlight intensity is present or if MP UV disinfection is employed. Significance and Impact of the Study:  With the increased use of UV disinfection, it is imperative that photoreactivation be taken into account in the design of reactors based on site‐specific conditions of temperature and light intensity exposure.

Ultraviolet (UV) disinfection is becoming increasingly popular as an alternative disinfection technology to chlorination in recent years. In this study, we investigated the photoreactivation of Escherichia coli following medium-pressure (MP) UV disinfection of synthetic water by a bench-scale collimated beam apparatus. The UV doses ranged from 1.6 -19.7 mWs/cm2 and photoreactivation was investigated for 6 hours under fluorescent light. In addition, chloramination was applied after UV disinfection to investigate its ability to control photoreactivation. It was found that photoreactivation occurred for all UV doses tested and the increase in bacteria numbers ranged from 0.04 to 1.35 log10. However, the degree of photoreactivation decreased with increased UV doses. Chloramination experiments revealed that the addition of 0.5 mg/l of monochloramine resulted in suppression of photoreactivation for 1 hour only. An increased monochloramine dose of 1 mg/l was found to prevent photoreactivation for the entire duration of the experiment. The results of this study have shown that photoreactivation occurs even after MP UV disinfection, although it is of a lesser extent at higher UV doses. This study has also established that secondary chloramination can effectively suppress and eliminate photoreactivation with a chloramine dose of 1 mg/l.

Ultraviolet (UV) light disinfection has increasingly been used as an alternative method to replace conventional chlorine disinfection as it has been found to be a more efficient disinfection method. As UV disinfection only damages the nucleic acids of the microorganisms to prevent replication, there is a possibility of microorganisms repairing the damage sites. As few studies have investigated the reactivation of microorganisms after exposure to medium-pressure UV disinfection, it is essential for reactivation related to medium-pressure UV disinfection to be studied as medium-pressure lamps are gaining in popularity. Besides, disinfection by-products (DBPs) produced by UV disinfection have been discovered recently and may serve as a carbon source in the finished water, resulting in regrowth of the bacteria. It is therefore important to know the regrowth potential of bacteria with the existence of DBPs. In this study, the repair and regrowth of Escherichia coli after UV disinfection were investigated. Results showed that E. coli underwent photo repair (up to 5 log under fluorescent light conditions) more significantly than dark repair (up to 0.8 log in terms of bacterial count increase). The repair was generally found to be higher at low doses. At the same UV dose, it seems medium-pressure UV irradiation is able to control the repair to a lesser extent. In addition, the bacterial regrowth potential was studied with the addition of DBPs typically found in UV processes, such as acetic acid and formaldehyde. The maximum increase in bacterial count was found to be 0.3 log. Generally, the level of regrowth was insignificant compared with the increase of bacterial count due to bacterial repair.

Photolyase activity following exposure to low-pressure (LP) and medium-pressure (MP) UV lamps was evaluated. MP UV irradiation resulted in a greater reduction in photolyase activity than LP UV radiation. The results suggest that oxidation of the flavin adenine dinucleotide in photolyase may have caused the decrease in activity.UV disinfection inactivates microorganisms through the formation of cyclobutane pyrimidine dimers in their DNA (4). However, these cyclobutane pyrimidine dimers may be removed via photoreactivation, which allows inactivated microorganisms to recontaminate the water. Studies have found that photoreactivation of Escherichia coli following medium-pressure (MP) UV disinfection is lower than that following lowpressure (LP) UV disinfection (9,14). A hypothesized explanation for this is damage to photolyase, because it contains a tryptophan-rich flavin adenine dinucleotide (FAD) cofactor that has a peak absorbance at 280 nm (4), a wavelength emitted by MP and not LP UV lamps (8). Nevertheless, there is currently no evidence to test the hypothesis, and research on this is limited. This study therefore aims to investigate the effects of LP and MP UV radiation on photolyase activity and to propose the mechanism for photoreactivation suppression by MP UV radiation reported previously.Photolyase was extracted and purified from E. coli containing plasmid with the phr gene in accordance with the methods described by Sancar and Sancar (12). One hundred microliters of photolyase (1 ϫ 10 Ϫ7 to 5 ϫ 10 Ϫ7 M) was dispensed onto a microcentrifuge tube cap and placed under a collimated beam apparatus (Calgon Carbon Corporation) with interchangeable LP and MP UV lamps. The UV intensity of the lamps was measured with a radiometer (IL1400A; International Light, Inc.) with a SED240 sensor. UV doses were calculated by the method of Bolton and Linden (1) and applied by varying the exposure times. Photolyase activity was determined using a spectrophotometric assay modified from that developed by Jorns et al. (5). Dimer substrate was mixed with irradiated photolyase, incubated in the dark for 3 to 5 min, and then exposed to 365 nm light (9 W) at a distance of 5 cm. Absorption spectra (250 nm to 320 nm) were taken at 2-min intervals. In other experiments, 5 mM dithiothreitol (DTT) was added to the mixture to investigate whether oxidation of the FAD caused the decrease in photolyase activity, and absorption spectra were taken at 1-min intervals. The increase in absorbance at 260 nm was plotted against the time, and the gradient of the straight-line portion was used to calculate the rate of dimer repair by applying the Beer-Lambert law, with ε 260 of thymine monomers taken to be 8.3 ϫ 10 3 M Ϫ1 cm Ϫ1 (5). The calculated rates of dimer repair by LP and MP UVirradiated photolyase based on absorbance changes in the substrate are summarized in Fig. 1. For LP UV-irradiated photolyase, the dimer repair rate was unaffected up to a dose of 10 mJ cm Ϫ2 (rates varied 3% between 0.467 and 0.480 M dimer M Ϫ1 photolyase min Ϫ1 ) and t...