Effects of UV Irradiation by Light Emitting Diodes on Heterotrophic Bacteria in Tap Water.

Ultraviolet light emitting diodes (UV-LEDs) are small mercury-free devices that can be installed at the point of use (POU) of water for disinfection. Considering that heterotrophic bacteria are of concern in drinking water systems, we applied a flow-through UV-LED apparatus to dechlorinated tap water, and determined the heterotrophic plate count (HPC) in samples after UV-LED exposure (UV+) compared to samples without UV-LED application (UV-). The UV+ and UV- samples were maintained at 20°C to track HPC profiles during storage for 7 days. It was confirmed that UV+ samples showed negative HPC or lower HPC than UV- for 5 days of storage after the flow-through test. HPC bacteria formed colonies with different morphological characteristics, and yellow colonies were closest to Novosphingobium sp., with 99% identity, while white and pale pink colonies were closest to Methylobacterium sp., with 99-100% identity, based on 16S rRNA gene sequences. White colonies became dominant in UV+, indicating that UV-LED exposure can select UV-resistant species such as Methylobacterium. This study shows the effects of UV-LED application on HPC bacteria in tap water and implies that future research is required on the significance and impacts of microbial selection by UV-LED exposure.