Sequential use of UV-LEDs irradiation and chlorine to disinfect waterborne fungal spores: Efficiency, mechanism and photoreactivation

The aggregation characteristics of Aspergillus spores under various conditions and the impact on LPUV inactivation: Comparisons with chlorine-based disinfection

Aggregation is the primary step prior to fungal biofilm development. Understanding the attributes of aggregation is of great significance to better control the emergence of waterborne fungi. In this study, the aggregation of Aspergills spores (A. flavus and A. fumigatus) under various salt, culture medium, and humic acid (HA) conditions was investigated for the first time, and the inactivation via low-pressure ultraviolet (LPUV) upon aggregated Aspergillus spores was also presented. The aggregation efficiency and size of aggregates increased over time and at low salt (NaCl and CaCl2) concentration (10 mM) while decreasing with the continuous increase of salt concentration (100 and 200 mM). Increasing the concentration of culture medium and HA promoted the aggregation of fungal spores. Spores became hydrated, swelled, and secreted more viscous substances during the growth period, which accelerated the aggregation process. Results also suggested that fungal spores aggregated more easily in actual water, posing a high risk of biohazard in real-life scenarios. Inactivation efficiency by LPUV decreased with higher aggregation degrees due to the protection from the damaged spores on the outer layer and the shielding of pigments in the cell wall. Compared to chlorine-based disinfection, the aggregation resulted in the extension of shoulder length yet neglectable change of inactivation rate constant under LPUV treatment. Further investigation of cell membrane integrity and intracellular reactive oxygen species was conducted to elucidate the difference in mechanisms between various techniques. This study provides insight into the understanding and controlling of the aggregation of fungal spores.

Formation of halonitromethanes from glycine during LED-UV265/chlorine disinfection

LED-UV265/chlorine is a promising alternative disinfection technology that emits mono-wavelength light for high germicidal efficiency. Halonitromethanes (HNMs) are highly cytotoxic and genotoxic disinfection byproducts that can be formed during LED-UV265/chlorine disinfection. Thus, this work aimed to investigate the HNMs formation from glycine (Gly) during LED-UV265/chlorine disinfection. The results indicated that the concentrations of chlorinated-HNMs (Cl-HNMs) increased first and then decreased as the reaction proceeded. Besides, the effects of operating parameters (UV intensity, free chlorine dosage, and pH) and coexisting ions (Cu2+ and Br-) on HNMs formation were investigated. It was found that the formation concentrations of Cl-HNMs increased with the increase of LED-UV265 intensity and free chlorine dosage but decreased with increased pH. The presence of Cu2+ promoted the formation of Cl-HNMs. The total concentration of HNMs (at 3 min) with adding 1.5 mg/L Cu2+ was 30.90% higher than that without Cu2+. Notably, nine species of HNMs were detected after adding Br-, and the total concentrations of HNMs were enhanced. Moreover, Cl-HNMs were gradually transformed into brominated (chlorinated)-HNMs and brominated-HNMs as Br- concentration increased. According to the findings, the possible formation mechanism of HNMs from Gly during LED-UV265/chlorine disinfection was deduced. Finally, it was demonstrated that the formation laws of HNMs from Gly in real water samples were basically consistent with those in simulated water. Insights obtained in this study help to comprehend the HNMs formation from Gly and provide strategies for controlling the production of HNMs during LED-UV265/chlorine disinfection.

Enhanced inactivation of Escherichia coli by ultrasound combined with peracetic acid during water disinfection

Peracetic acid (PAA) is a desirable disinfectant for municipal wastewater because of its potent disinfection performance and limited toxic by-products. This study explored the efficiency and mechanism of Escherichia coli inactivation by PAA combined with ultrasound simultaneously (ultrasound + PAA) or (ultrasound → PAA) sequentially. The result showed that 60 kHz ultrasound combined with PAA sequentially (60 kHz → PAA) had excellent inactivation performance on E. coli, up to 4.69-log₁₀. The result also showed that the increase of pH and humic acid concentration in solution significantly reduced the inactivation efficiency of 60 kHz → PAA treatment. We also observed that the increase of temperature was beneficial to the disinfection, while anions (Cl^-; HCO₃^-) had little effect. With 60 kHz → PAA, the PAA and the synergism between PAA and ultrasound played major contribution to the inactivation, which we assumed might be due to both the diffusion of PAA into the cells and the damage to the cytomembrane by ultrasound, as evidenced through the laser confocal microscopy (LSCM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The inactivation mechanism involved the destruction of cell membrane and loss of intracellular material. Empirically, 60 kHz → PAA was found to be effective for the inactivation of E. coli in actual wastewater, and the regrowth potential of E. coli treated by 60 kHz → PAA was significantly lower than that treated only by PAA.

Occurrence and control of fungi in water: New challenges in biological risk and safety assurance

Recently, the contamination of fungi in water has aroused widespread concern, which will pose a threat to water quality and safety, and raise diseases risk in the immunocompromised individuals. In this review, the characteristics and different physiological state of fungi in water are summarized. A comprehensive evaluation of the control efficiency and mechanism of waterborne fungi by the commonly used disinfection methods is provided as well. During the disinfection processes of chlorine, chlorine dioxide, chloramine and advanced disinfection processes (ADPs) such as O₃-based ADPs and UV-based ADPs, the fungal spores firstly lost their culturability, followed by membrane integrity, and the intracellular reactive oxygen species level increased at the same time, eventually the fungal spores were completely inactivated. The security strategies of drinking water against the contamination of fungi are also discussed in terms of water sources, water treatment plants and pipe network. Finally, future researches need to be explored are proposed: the rapid detection methods, the production laws and control of mycotoxin, and the outbreak conditions of fungi in water. Specifically, exploring efficient, safe and economical technologies, especially ADPs, is still the main direction in the disinfection of fungi in future studies. This review can offer a comprehensive understanding on the occurrence and control of fungi in water to fill the knowledge gap and provide guidance for the future research.

UV C Light from a Light-Emitting Diode at 275 Nanometers Shortens Wound Healing Time in Bacterium- and Fungus-Infected Skin in Mice

Due to the changes in pathogenic species and the absence of research on topical skin antibiotics, the therapy of skin and soft tissue infections (SSTIs) is facing more and more severe challenges. It is particularly urgent to look for alternative therapies without induction of drug resistance. UV C (UVC) light within the range of 200 to 280 nm is one of the most common techniques used to kill and/or inactivate pathogenic microorganisms. However, the traditional most commonly used wavelength of 254 nm irradiated from a low-pressure mercury lamp is hazardous to human health, being both carcinogenic and damaging to eye tissues, which limits its applications in vivo. This research aimed to investigate the antimicrobial properties and influence of 275-nm UVC light from a light-emitting diode (UVC-LED light) on wound healing time. Five bacteria, three fungi, and scalded-mouse models combined with SSTIs were used to evaluate the antimicrobial effect in vitro and in vivo. 275-nm UVC-LED light inactivated both bacteria and fungi with a very short irradiation time in vitro and induced neither DNA damage nor epidermal lesions in the mice's skin. Furthermore, in mouse models of SSTIs induced by either methicillin-resistant Staphylococcus aureus (MRSA) or Candida albicans, the 275-nm UVC-LED light showed significant antimicrobial effects and shortened the wound healing time compared with that in the no-irradiation group. UVC-LED light at 275 nm has the potential to be a new form of physical therapy for SSTIs. IMPORTANCE As a common clinical problem, the therapy of SSTIs is facing growing challenges due to an increase in the number of drug-resistant bacteria and fungi. UV C (UVC) light sterilization has been widely used in all aspects of daily life, but there are very few reports about in vivo therapy using UVC light. It is well known that prolonged exposure to UVC light increases the possibility of skin cancer. In addition, it is also very harmful for eyes. UV irradiation with 254-nm UVC light can cause corneal damage, like thinning of the corneal epithelial layer, superficial punctate keratitis, corneal erosion, etc. In this study, we focused on looking for a more accessible light source and safer UVC wavelength, and 275-nm UVC LED light was chosen. We investigated its applicability for SSTIs therapy with relative skin safety and expected that it could be used as a new physical therapy method for SSTIs.

Change of algal organic matter under different dissolved oxygen and pressure conditions and its related disinfection by-products formation potential in metalimnetic oxygen minimum

Most of the reservoirs or lakes will form a metalimnetic oxygen minimum (MOM) with the characterization of a substantial fraction of dissolved oxygen (DO) depleted below the epilimnion. The effect of intracellular organic matter (IOM) of algal cells transformed under MOM conditions is completely different from that of the original IOM on water quality. In this study, the IOM changes of Microcystic aeruginosa under different MOM conditions and its related disinfection by-products formation potentials (DBPFPs) were investigated by changing the pressure and DO concentration of MOM. Total Fmax increased slightly and then decreased under different pressure conditions, finally decreasing by no more than 22.0%. Under aerobic condition, dissolved organic carbon (DOC) and total Fmax decreased significantly, and decreased by 60.4% and 38.8% within the first 2 days. The results of specific UV absorbance (SUVA) and UV₂₅₀/UV₃₆₅ indicated that aromatic compounds and average molecular weight of IOM were gradually increased under different MOM conditions. The total DBPFPs increased firstly and then decreased under different pressure conditions, and finally decreased by 26.2%-33.1%. The decrease of total DBPFPs was significantly higher under aerobic condition than that under anoxic condition, which finally decreased by 64.5%. Redundancy analysis showed that the fluorescence parameter (protein-like and humic-like fluorescence) could be expected as an index to predict the DBPFPs. Moreover, the results revealed that with the decrease of DO, the activity and diversity of natural microbial consortium decreased, which prevented the further degradation and utilization of organic matter by natural microbial consortium. Therefore, lower DO was a key player for the deterioration of water quality under MOM conditions.

Enhanced solar inactivation of fungal spores by addition of low-dose chlorine: Efficiency and mechanism

This work demonstrated that the solar inactivation of fungal spores was enhanced by addition of low-dose chlorine. Although the effect of low-dose chlorine alone (2.0 mg/L) on culturability of fungal spores was negligible, the solar/chlorine inactivation on fungal spores performed better than solar alone inactivation, with a lower shoulder length and a higher maximum inactivation rate constant. The enhanced inactivation of Aspergillus niger can be ascribed to the membrane oxidation by chlorine, and the enhanced inactivation of Penicillium polonicum can be ascribed to the membrane oxidation by chlorine and ·OH (·OH plays a major role). The oxidization by chlorine and ·OH led to an increase in membrane permeability of fungal spores, which enhanced the solar inactivation, resulting in an increase in intracellular ROS and more serious morphological damage. Due to the presence of background substances such as dissolved organic matter and metal ions (Fe²⁺, Mn²⁺, etc.), the inactivation efficiency in real water matrices was decreased. The main disinfection by-products (DBPs) produced in the inactivation of fungal spores in chlorine alone and solar/chlorine treatments were dichloroacetic acid, trichloroacetic acid, trichloroacetone and trichloromethane. Generally, DBPs formation in solar/chlorine treatment was lower than those in chlorine alone treatment. Moreover, the regrowth potential of the two genera of fungal spores in R2A medium could be inhibited by adding low-dose chlorine.

Inactivation and subsequent reactivation of Aspergillus species by the combination of UV and monochloramine: Comparisons with UV/chlorine

Ultraviolet (UV)/monochloramine (NH₂Cl) as an advanced oxidation process was firstly applied for Aspergillus spores inactivation. This study aims to: i) clarify the inactivation and photoreactivation characteristics of UV/NH₂Cl process, ii) compared with UV/Cl₂ in inactivation efficiency, photoreactivation and energy consumption. The results illustrated that UV/NH₂Cl showed better inactivation efficiency than that of UV alone and UV/Cl₂, and could effectively control the photoreactivation. For instance, the inactivation rates for Aspergillus flavus, Aspergillus niger and Aspergillus fumigatus in the processes of UV/NH₂Cl (2.0 mg/L) was 0.034, 0.030 and 0.061 cm²/mJ, respectively, which were higher than that of UV alone (0.027, 0.026 and 0.024 cm²/mJ) and UV/Cl₂ (0.023, 0.026 and 0.031 cm²/mJ). However, there was no synergistic effect for Aspergillus flavus and Aspergillus fumigatus. As for Aspergillus niger, the best synergistic effect can reach 1.86-log₁₀. This may be due to their different resistance to disinfectants, which were related to the size, an outer layer of rodlets (hydrophobins) and pigments. After UV/NH₂Cl inactivation, the degree of cell membrane damage and intracellular reactive oxygen species were higher than that of UV alone. UV/NH₂Cl had the advantages of high inactivation efficiency and inhibition of photoreactivation, which provides a new entry point for the disinfection of waterborne fungi.