Differences in UV-C LED Inactivation of Legionellapneumophila Serogroups in Drinking Water

Abstract

Legionella pneumophila (Lp) is an opportunistic pathogen that causes respiratory infections primarily through inhalation of contaminated aerosols. Lp can colonize premise plumbing systems due to favorable growth conditions (e.g., lower disinfectant residual, stagnation, warm temperatures). UV-C light-emitting diodes (UV-C LEDs) are an emerging water treatment technology and have been shown to effectively inactivate waterborne pathogens. In this study, the inactivation of four Lp strains (three clinical sg1, 4, and 6; and one sg1 drinking water (DW) isolate) was evaluated using a UV-C LED collimated beam at three wavelengths (255, 265, and 280 nm) and six fluence rates (0.5-34 mJ/cm²). Exposure to 255 nm resulted in higher log reductions at the lower fluences compared to exposures at 265 and 280 nm. Efficacy testing was also performed using a UV-C LED point-of-entry (POE) flow-through device. Based on the log inactivation curves, at 255 nm, the sg4 and sg6 clinical isolates were more susceptible to inactivation compared to the two sg1 isolates. However, at 265 and 280 nm, the sg1 and sg4 clinical isolates were more resistant to inactivation compared to the sg6 clinical and sg1 DW isolates. Differential log reductions were also observed using the POE device. Results indicate that although UV-C LED disinfection is effective, variations in Lp inactivation, wavelengths, and technology applications should be considered, especially when targeting specific isolates within premise plumbing systems.

Keywords: UV-C LED disinfection; decontamination; mechanisms; opportunistic pathogens; potable water; premise plumbing; treatment.

Figure 1

Graphical depiction of the UV-C LED POE device test set-up. (1) Source of drinking water delivered via tygon tubing, (2) flow meter, (3) influent sampling port, (4) pump used to deliver the Lp inoculum into the water with a backflow preventer (tube clamp), (5) static mixer, (6) pre-treatment sampling port, (7) UV-C LED POE device, (8) post-treatment sampling port, (9) 55-gallon biological liquid waste collector. See Supplementary Figure S1 for a schematic and image of the testing set-up.

Figure 2

Inactivation of Lp strains using the UV-C LED collimated beam. Lp sg 1 (A), sg 1 DW isolate (B), sg 4 (C), and sg 6 (D) were exposed to various UV fluences at 255 nm (black open circles), 265 nm (green squares), and 280 (blue triangles) as described in Materials and Methods. Data (mean ± standard deviation) are representative of three replicates for each strain. The limit of detection of 0.7 log₁₀ CFU/mL is indicated by the dotted line. Statistical significance when compared to all other strains are denoted as ⁺ for p < 0.05, * for p < 0.01, and ** for p < 0.001. See also Supplemental Materials Table S1.

Figure 3

Lp and HPC log reductions using the UV-C LED POE device. Data represent the mean ± SD_LR log₁₀ CFU mL⁻¹ reductions in Lp sg 1 (white bars), sg 1 DW isolate (light grey bars), sg 4 (dark grey bars), and sg 6 (black bars) and the reduction in HPC levels in the drinking water source during each of the Lp strain experimental runs. SD_LR was calculated as described in Section 2.6. Data are representative of five replicates for each strain. Statistical significance when compared to all other strains is denoted as ** for p < 0.001. See Supplemental Materials Table S2.