Microbial inactivation kinetics of UV LEDs and effect of operating conditions: A methodological critical analysis

Inactivation Kinetics of Alicyclobacillus acidoterrestris Spores and Determination of Spore Germicidal Fluences Under UV-C Treatment

The aim of this study is to measure the UV-C inactivation kinetics and determine the fluences required for incremental inactivation of Alicyclobacillus acidoterrestris (AAT). Spores from five strains of AAT (ATCC 49025, DSM 2498, VF, SAC, and WAC) were suspended in clear phosphate-buffered saline (PBS) and individually treated with UV-C doses up to 100 mJ/cm2. A collimated beam device emitting UV-C at 254 nm (from a monochromatic low-pressure mercury lamp [LPM]) and at 268 nm (from UV light-emitting diodes [UV-LEDs]) was used for UV treatments. The log reduction from each treatment was plotted against the UV-C fluence. Curve fitting using the GInaFiT tool for Excel was attempted using both linear and nonlinear regression models. The goodness-of-fit and model performances, assessed using Akaike's Information Criterion and Bayesian Information Criterion, revealed that the Weibull model provided a better fit for the inactivation data and was thus used to determine UV-C doses required for 1-log inactivation and incremental log inactivation. Similar AAT spore inactivation efficacy was observed at both 254 and 268 nm. A UV-C dose of 100 mJ/cm2 at 254 nm inactivated >4-log CFU/mL, while at 268 nm, a 3.7-5.08-log CFU/mL reduction was observed for AAT strains ATCC 49025, DSM 2498, WAC, and VF. Among the five strains of AAT tested, spores of WAC demonstrated greater resistance, requiring UV-C doses of 2.76 mJ/cm2 and 100 mJ/cm2 for 1-log (D10-value) and 4-log inactivation at 254 nm, and 5.89 mJ/cm2 and >100 mJ/cm2 at 268 nm. In contrast, spores of SAC showed greater sensitivity, with UV-C doses of 1.87 mJ/cm2 and 47.92 mJ/cm2 required for 1-log and 4-log inactivation at 254 nm, and 6.20 mJ/cm2 and 44.61 mJ/cm2 at 268 nm. This study lays the foundation for designing a successful UV-based nonthermal pasteurization system.

A comprehensive review on ultraviolet disinfection of spices and culinary seeds and its effect on quality

Spices and culinary seeds, valued for their flavor and aroma, pose unique challenges for disinfection, as heat treatments are often unsuitable. Their raw consumption increases the risk of contamination, particularly with Salmonella spp. Thermal treatments are widely used for food disinfection due to their effectiveness in inactivating bacteria. However, these methods often degrade the nutritional and sensory qualities of food. Ultraviolet (UV) light, however, is a promising nonthermal technique that balances microbial inactivation and food quality preservation. This review employed a systematic approach to evaluate the effects of UV treatments, both alone and in combination with other techniques, on the microbiological safety and chemical composition of spices and culinary seeds. UV treatments have been shown to effectively inactivate bacteria, molds, and mycotoxins without triggering the same chemical reactions that reduce the quality of plant-based foods. Some studies have even suggested improvements in nutritional parameters following UV exposure, such as the increase of antioxidant activity or total phenolic content. However, inconsistencies in study quality limit the strength of current conclusions, and further research is needed. Critical areas for future investigation include scaling UV reactors, combining treatments, exploring UV-LED technology, conducting sensory analyses, and studying the inactivation of bacterial spores and mycotoxins.

Application of UV LEDs to inactivate antibiotic resistant bacteria: Kinetics, efficiencies, and reactivations

Unregulated antibiotic use has led to the proliferation of antibiotic-resistant bacteria (ARB) in aquatic environments. Ultraviolet light-emitting diodes (UV LEDs) have evolved as an innovative technology for inactivating microorganisms offering several advantages over traditional mercury lamps. This research concentrated on utilizing UV LEDs with three distinct wavelengths (265 nm, 275 nm, and 285 nm) to inactivate E. coli DH10β encoding the ampicillin-resistant blaTEM-1 gene in its plasmid. Non-linear models, such as Geeraerd's and Weibull, provided more accurate characterization of the inactivation profiles than the traditional log-linear model due to the incorporation of both biological mechanisms and a deterministic approach within non-linear models. The inactivation rates of ARB were higher than antibiotic-sensitive bacteria (ASB) when subjected to UV LEDs. The highest inactivation rates were observed when all microorganisms were exposed to 265 nm. Photoreactivation emerged as the primary mechanism responsible for repairing DNA damage induced by UV LEDs. 285 nm showed the highest reactivation efficiencies for ARB under different fluences. At higher fluences, both 265 and 275 nm displayed similar effectiveness in suppressing reactivation, while at lower fluences, 275 nm exhibited better efficacies in controlling the reactivation. Therefore, the inhibition of reactivation was influenced by the extent of damage incurred to both DNA and enzymes. In nutrient-poor media (0.9 % NaCl), ASB did not exhibit any reactivation potential. However, the addition of Luria-Bertani (LB) broth promoted the reactivation of ASB. Lower fluence rate was more beneficial at 265 nm whereas higher fluence rates were more effective for longer wavelengths. The inactivation of ARB was enhanced by dissolved organic carbon (DOC) at low fluences. However, the removal of ARB was reduced due to the presence of DOC at higher fluences. The highest energy demand for ARB inactivation was reported at 285 nm. ENVIRONMENTAL IMPLICATION: The excessive and unregulated utilization of antibiotics has emerged as a significant issue for public health. This paper presents a comprehensive analysis of the effectiveness of UV LEDs, an emerging technology, in the inactivation of antibiotic-resistant bacteria (ARB). This research paper explores the kinetics of UV LEDs with different wavelengths to inactivate ARB along with the reactivation efficiencies. This research work also explores the impact and relevant mechanisms of the impact of dissolved organic carbon (DOC) on the inactivation of ARB by UV LEDs.

Synergistic effect of UV-A and UV-C light is traced to UV-induced damage of the transfer RNA

UV light emitting diodes (LEDs) are considered the new frontier of UV water disinfection. As UV technologies continue to evolve, so does the need to understand disinfection mechanisms to ensure that UV treatment continues to adequately protect public health. In this research, two Escherichia coli (E. coli) strains (the wild type K12 MG1655 and K12 SP11 (ThiI E342K)) were irradiated with UV-C at 268 nm both independently and after exposure to UV-A (365 nm). A synergistic effect was found on the viability of the wild type E. coli K12 strain when UV-A irradiation was applied prior to UV-C. Sublethal UV-A doses, which had a negligible effect on cell viability alone, enhanced UV-C inactivation by several orders of magnitude. This indicated a specific cellular response mechanism to UV-A irradiation, which was traced to direct photolysis of the transfer RNA (tRNA), which are critical links in the translation of messenger RNA to proteins. The wild type K12 strain MG1655, containing tRNAs with a thiolated uridine, directly absorbs the UV-A light, which leads to a reduction in protein synthesis, making them more susceptible to UV-C induced damage. However, the K12 strain SP11 (ThiI E342K), with a point mutation in the thiI gene that prevents a post-transcriptional modification of tRNA, experienced less inactivation upon subsequent irradiation by UV-C. The growth rate of cells, which was inhibited by sublethal UV-A doses, was not inhibited in this mutant strain with the modified tRNA. Time-lapse microscopy with microfluidics showed that sub-lethal UV-A caused a transient, reversible, growth arrest in E. coli. However, once the growth resumed, the cell division time resembled that of unirradiated cells. Damage induced by UV-A impaired the recovery of damage induced by UV-C. Depending on the UV-A dose applied, the synergistic effect remained even when there was a time delay of several hours between UV-A and UV-C exposures. The effect of sublethal UV-A was reversible over time; therefore, the synergistic effect was strongest when UV-C was applied immediately after UV-A. Combining UV-A and UV-C irradiation may serve as a practical tool to increase UV disinfection efficacy, which could potentially reduce costs while still adequately protecting public health.

Germicidal efficacy of continuous and pulsed ultraviolet-C radiation on pathogen models and SARS-CoV-2

Ultraviolet radiation's germicidal efficacy depends on several parameters, including wavelength, radiant exposure, microbial physiology, biological matrices, and surfaces. In this work, several ultraviolet radiation sources (a low-pressure mercury lamp, a KrCl excimer, and four UV LEDs) emitting continuous or pulsed irradiation were compared. The greatest log reductions in E. coli cells and B. subtilis endospores were 4.1 ± 0.2 (18 mJ cm-2) and 4.5 ± 0.1 (42 mJ cm-2) with continuous 222 nm, respectively. The highest MS2 log reduction observed was 2.7 ± 0.1 (277 nm at 3809 mJ cm-2). Log reductions of SARS-CoV-2 with continuous 222 nm and 277 nm were ≥ 3.4 ± 0.7, with 13.3 mJ cm-2 and 60 mJ cm-2, respectively. There was no statistical difference between continuous and pulsed irradiation (0.83-16.7% [222 nm and 277 nm] or 0.83-20% [280 nm] duty rates) on E. coli inactivation. Pulsed 260 nm radiation (0.5% duty rate) at 260 nm yielded significantly greater log reduction for both bacteria than continuous 260 nm radiation. There was no statistical difference in SARS-CoV-2 inactivation between continuous and pulsed 222 nm UV-C radiation and pulsed 277 nm radiation demonstrated greater germicidal efficacy than continuous 277 nm radiation. Greater radiant exposure for all radiation sources was required to inactivate MS2 bacteriophage. Findings demonstrate that pulsed irradiation could be more useful than continuous UV radiation in human-occupied spaces, but threshold limit values should be respected. Pathogen-specific sensitivities, experimental setup, and quantification methods for determining germicidal efficacy remain important factors when optimizing ultraviolet radiation for surface decontamination or other applications.