The protective role and mechanism of melanin for Aspergillus niger and Aspergillus flavus against chlorine-based disinfectants

A Gene Cassette Vd276-280 in Verticillium dahliae Contains Two Genes that Affect Melanized Microsclerotium Formation and Virulence

Verticillium dahliae is a soilborne phytopathogenic fungus causing Verticillium wilt on hundreds of plant species. Several sequenced genomes of V. dahliae are available, but functional characterization of most genes has just begun. Based on our previous comparison of the transcriptome from the wild-type and ΔVdCf2 strains, a significant upregulation of the gene cassette, Vd276-280, in the ΔVdCf2 strain was observed. In this study, the functional characterization of the Vd276-280 gene cassette was performed. Agrobacterium-mediated knockout of this gene cassette in V. dahliae significantly inhibited conidiation, melanized microsclerotium formation in the mutant strains, and their virulence toward cotton. Furthermore, deletion of individual genes in the Vd276-280 gene cassette identified that the disruption of VDAG_07276 and VDAG_07280 delayed microsclerotium formation, inhibited conidiation, and reduced virulence toward cotton. Our data suggest that VDAG_07276 and VDAG_07280 in the Vd276-280 gene cassette mainly act as positive regulators of development and virulence in V. dahliae.

Enhanced inactivation of Aspergillus niger biofilms by the combination of UV-LEDs with chlorine-based disinfectants

The presence of pathogenic fungal biofilms in drinking water distribution systems poses significant challenges in maintaining the safety of drinking water. This research delved into the formation of Aspergillus niger (A. niger) biofilms and evaluated their susceptibility to inactivation using combinations of ultraviolet light emitting diodes (UV-LEDs) with chlorine-based disinfectants, including UV-LEDs/chlorine (Cl2), UV-LEDs/chlorine dioxide (ClO2), and UV-LEDs/chloramine (NH2Cl) at 265 nm, 280 nm and 265/280 nm. Results indicated that A. niger biofilms reached initial maturity within 24 h, with matured three-dimensional filamentous structures and conidiospores by 96 h. UV-LEDs combined with chlorine-based disinfectants enhanced A. niger biofilm inactivation compared to UV-LEDs alone and low-pressure UV combined with chlorine-based disinfectants. At an UV fluence of 400 mJ/cm2, log reductions of UV265, UV280, and UV265/280 combined with chlorine-based disinfectants were 2.95-fold, 3.20-fold, and 2.38-fold higher than that of UV265, UV280, and UV265/280, respectively. During the inactivation, A. niger biofilm cells experienced increased membrane permeability and intracellular reactive oxygen species levels, resulting in cellular apoptosis. Extracellular polymeric substances contributed to the higher resistance of biofilms. Regarding electrical energy consumption, the order was: UV-LEDs/ClO2 > UV-LEDs/NH2Cl > UV-LEDs/Cl2. These findings provide insights into the effective utilization of UV-LEDs for fungal biofilm disinfection.

Efficacy Evaluation of Chlorine Dioxide and Hypochlorous Acid as Sanitisers on Quality and Shelf Life of Atlantic Salmon (Salmo salar) Fillets

Microbial contamination during seafood processing can often lead to a reduction in shelf life and the possibility of food-borne illnesses. Sanitisation with chlorine-based products during seafood processing is therefore sometimes undertaken. This study compared the effects of two sanitisers, chlorine dioxide (ClO2) and hypochlorous acid (HOCl) at their suggested concentration (5 ppm and 10 ppm; 50 ppm and 100 ppm respectively), on physical, chemical, and microbial qualities of Atlantic salmon (Salmo salar) fillets throughout 7 days of simulated retail display refrigeration. Parameters used for assessment included quality index (QI), drip loss, colour, texture, histology, total volatile base nitrogen (TVB-N), lipid oxidation (malonaldehyde, MDA), pH, and total viable count changes. Results indicated that whilst drip loss increased over the storage time, day 4 and 7 drip loss in both sanitisers decreased significantly compared with the control. There was a linear relationship (R > 0.70) between QI and storage time in all treatments, particularly in regard to skin brightness, flesh odour, and gaping parameters, but treatment differences were not present. Texture parameters including gumminess, chewiness, and hardness increased over time in the control whilst both sanitiser treatments seemed to provide protective effects against texture hardening during storage. The observed softening effects from the sanitiser treatments were aligned with microstructural and cytological changes in the histology results, as evidenced by a reduced fibre-fibre adhesion, myodigeneration, and an increase in interfibrillar space over storage time. Colour, especially chroma (C*), was shown to decrease over time in control, whereas insignificant protective effects were observed in both sanitiser treatments at day 7. Irrespective of treatment and storage time, MDA levels exceeded the acceptable limit on all days, whilst TVB-N levels were below the critical limit. Although pH was influenced by treatment and storage time, the pH was within the normal range. Microbiological results showed that with sanitiser addition, TVC was below the permissible level (106 CFU/g) until day 4 but ClO2 ice (5 ppm), ClO2 (10 ppm), and HOCl (100 ppm) treated fillets all exceeded the limit on day 7. The mixed results on the effect of sanitiser addition on fillet quality and shelf life suggested that further investigation on pathogen reduction, sanitiser introductory method, as well as testing the same treatments in low-fat fish models would be recommended.

Application of chlorine dioxide and its disinfection mechanism

Chlorine dioxide (ClO2) is a strong oxidizing agent and an efficient disinfectant. Due to its broad-spectrum bactericidal properties, good inactivation effect on the vast majority of bacteria and pathogenic microorganisms, low resistance to drugs, and low generation of halogenated by-products, chlorine dioxide is widely used in fields such as water purification, food safety, medical and public health, and living environment. This review introduced the properties and application status of chlorine dioxide, compared the action mode, advantages and disadvantages of various disinfectants. The mechanism of chlorine dioxide inactivating bacteria, fungi and viruses were reviewed. The lethal target of chlorine dioxide to bacteria and fungi is to destroy the structure of cell membrane, change the permeability of cell membrane, and make intracellular substances flow out, leading to their death. The lethal targets for viruses are the destruction of viral protein capsids and the degradation of RNA fragments. The purpose of this review is to provide more scientific guidance for the application of chlorine dioxide disinfectants.

Formation of filamentous fungal biofilms in water and the transformation of resistance to chlor(am)ine disinfection

Biofilms are composed of complex multi-species in nature, potentially threatening drinking water safety. In this work, the formation of single- and multi-species fungal biofilms formed by Aspergillus niger (A. niger) and Aspergillus flavus (A. flavus), and the inactivation of mature biofilms using chlor(am)ine were firstly investigated. Results revealed that the antagonistic interaction occurred between A. niger and A. flavus. Chloramination at 20 mg/L for 30 min achieved 74.74 % and 76.04 % inactivation of A. flavus and multi-species biofilm, which were 1.69- and 1.84-fold higher than that of chlorine at the same condition. However, no significant difference was observed in the inactivation of A. niger biofilm between chlorine and monochloramine disinfection due to the lower amount of extracellular polymeric substance produced by it (p > 0.05). The inactivation of biofilm by monochloramine fitted the Weibull model well. According to the Weibull model, the monochloramine resistance of biofilm were as follows: A. flavus > multi-species > A. niger biofilm. Besides, an increase in reactive oxygen levels, damage of cell membrane, and leakage of intracellular substances in biofilms were observed after chlor(am)ination. More intracellular polysaccharides and proteins were leaked in chloramination inactivation (p < 0.05). This study provides important implications for controlling fungal biofilm.

Solar/ClO2 system inactivates fungal spores in drinking water: Synergy, efficiency and mechanisms

The risk of fungal pollution in drinking water has been paid attention. Solar/chlorine dioxide (ClO2) combined system is an environment-friendly, economical and efficient disinfection method, especially for countries and regions that are economically backward and still exposed to unsafe drinking water. In this paper, the kinetics, influencing factors, mechanism and regrowth potential of inactivated Aspergillus niger (A. niger) spores by solar/ClO2 were reported for the first time. The inactivation curve can be divided into three stages: instant inactivation within 1-2 min, slow linear inactivation and finally a tail. The synergistic factors produced by solar/ClO2 in terms of log reduction and maximum inactivation rate were 1.194 and 1.112, respectively. The inhibitory effect on the regrowth of A. niger spores inactivated by solar/ClO2 was also stronger than that by ClO2 alone. Strongly oxidizing reactive species produced by solar/ClO2 accelerated the accumulation of endogenic reactive oxygen species (ROS) caused by oxidation stress of A. niger spores, improving the inactivation ability of the system. The inactivation order of A. niger spores was: loss of culturability, accumulation of intracellular ROS, loss of membrane integrity, leakage of intracellular species and change of morphology. The inactivation performance of solar/ClO2 was better than solar/chlor(am)ine according to the comparison of inactivation efficiency and regrowth potential. Results also suggested that solar/ClO2 process was more suitable for the treatment of ground water sources.

Advanced oxidation process/coagulation coupled with membrane distillation (AOP/Coag-MD) for efficient ammonia recovery: Elucidating biofouling control performance and mechanism

The inadequate understanding of the biofouling formation mechanism and the absence of effective control have inhibited the commercial application of membrane distillation (MD). In this study, an advanced oxidation process (AOP)/coagulation-coupled (Coag) membrane distillation system was proposed and exhibited the potential for MD ammonia recovery (recovery rate: 94.1%). Extracellular polymeric substances (EPS) and soluble microbial products (SMP) components such as humic acid and tryptophan-like proteins were disrupted and degraded in the digestate. The curtailment and sterilizing efficiency of AOP on biofilm growth was also verified by optical coherence tomography (OCT) in situ real-time monitoring and confocal laser scanning microscopy (CLSM). Peroxymonosulfate (PMS) was activated to generate sulfate (SO4•-) and hydroxyl radicals (HO•), which altered the microbial community. After oxidative treatment, 16 S rRNA sequencing indicated that the dominant phylum of the microbial community evolved into Firmicutes. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis demonstrated that free radicals produced by PMS could disrupt cells' signaling molecules and interactions. In conjunction with these analyses, the mechanisms of response to free radical attack by Gram-negative bacteria, Gram-positive bacteria, and fungi were revealed. This research provided new insights into the field of membrane fouling control for membrane technology resource recovery processes, broadening the impact of AOP applications on microbiological response and fate in the environment.

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.

Solar-driven strongly coupled plasmonic Au nanoarrays on mesoporous silica nanodisks enable selective fungal and bacterial inactivation in well water

Well water is an important water source in isolated rural areas but easily suffers from microbial contamination. Herein, we anchored periodic Au nanoarrays on mesoporous silica nanodisks (Au-MSN) to fabricate a solar-driven nano-stove for well water disinfection. The solar/Au-MSN process completely inactivated 3.98, 6.55, 7.11 log10 cfu/mL, and 3.37 log10 pfu/mL of Aspergillus niger spores, Escherichia coli, chlorine-resistant Spingopyxis sp. BM1-1, and bacteriophage MS2 within 5 min, respectively. Moreover, the complete inactivation of various microorganisms (even at a viable but nonculturable state) was achieved in the flow-through reactor under natural solar light in real well water matrixes. Thorough characterizations and theoretical simulations verified that the densely anchoring strategy of Au-MSN's nanoarray worked on broadband absorption via the photon confinement effect, and trace amounts of Au can induce strong electromagnetic fields and collective localized heating. The resulting surge of 1O2 and heat synergically destroyed membranes, dysfunction cellular self-defense and metabolic system, induced intracellular oxidative stress, and ultimately inactivated microorganisms. Additionally, the 1O2-dominated oxidation and cell adhesion facilitated the selective disinfection in real well water matrixes. This study provides a cost-effective and practical solution for efficient well water disinfection, which assists isolated rural areas in getting safe drinking water.

Kinetics and mechanism of sulfate radical-and hydroxyl radical-induced disinfection of bacteria and fungal spores by transition metal ions-activated peroxymonosulfate

Peroxymonosulfate(PMS)-based advanced oxidation process have been recognized as efficient disinfection processes. This study comprehensively investigated the role of sulfate radical (SO4•-) and hydroxyl radical (•OH)-driven disinfection of bacteria and fungal spores by the PMS/metals ions (Me(II)) systems and modeled the CT value based on the relationship between survival and ∫[Radical]dt, with the aim to provide an accurate and quantitative kinetic data of inactivation processes. The results indicated that •OH played a more central role than SO4•- in the inactivation process, and bacteria were more vulnerable to radical attack than fungal spores due to the differences in antioxidant mechanisms and external structures. The k value of •OH -induced inactivation of E. coli was approximately 3-fold higher than that of A. niger, and the shoulder length of •OH -induced inactivation of E. coli was closely 52-fold shorter than that of A. niger after treated with the PMS/Co(II) system. The morphological and biochemical changes revealed that PMS/Me(II) treatment caused membrane damage, intracellular ROS accumulation and esterase activity loss in microorganisms. This study significantly improved the understanding of the contribution of radicals in the process of microbial inactivation by PMS/Me(II) and would provide important implications for the further development of technologies to cope with the highly resistant fungal spores in drinking water.

Inactivation of fungal spores by performic acid in water: Comparisons with peracetic acid

Performic acid (PFA) has received increasing attention in water disinfection due to its high disinfection efficiency and fewer formation of disinfection by-products. However, the inactivation of fungal spores by PFA has not been investigated. In this study, the results showed that the log-linear regression plus tail model adequately described the inactivation kinetic of fungal spores with PFA. The k values of A. niger and A. flavus with PFA were 0.36 min^-1 and 0.07 min^-1, respectively. Compared to peracetic acid, PFA was more efficient in inactivating fungal spores and caused more serious damage on cell membrane. Compared to neutral and alkaline conditions, acidic environments demonstrated a greater inactivation efficiency for PFA. The increase of PFA dosage and temperature had a promoting effect on the inactivation efficiency of fungal spores. PFA could kill the fungal spores by damaging cell membrane and penetration of cell membranes. In real water, the inactivation efficiency declined as a result of the existence of background substances such as dissolved organic matter. Moreover, the regrowth potential of fungal spores in R2A medium were severely inhibited after inactivation. This study provides some information for PFA to control fungi pollution and explores the mechanism of PFA inactivation.

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.

Pre-exposure of peracetic acid enhances its subsequent combination with ultraviolet for the inactivation of fungal spores: Efficiency, mechanisms, and implications

Waterborne fungi pose a potential threat to water supply safety due to their high resistance to disinfectants. Peracetic acid, as a promising alternative disinfectant to chlorine, has attracted increasing attention in water treatment. In this study, the inactivation of two dominant fungal species (Aspergillus niger and Aspergillus flavus) by sequential application of peracetic acid and ultraviolet (PAA-UV/PAA) was reported for the first time. Results revealed that the pre-exposure of PAA could facilitate the subsequent process of UV/PAA combination and shorten the lag phase in fungi inactivation. After 10 min of PAA pre-exposure, PAA-UV/PAA achieved 3.03 and 2.40 log inactivation of Aspergillus niger and Aspergillus flavus, which were 2- and 4.3-fold higher than that of direct UV/PAA under the same UV and PAA doses. PAA-UV/PAA disinfection also exhibited a stronger regrowth inhibition for incompletely inactivated fungal spores than direct UV/PAA. The increase of pH (5.0-9.0) and humic acid concentration (1.0-5.0 mg L ^- 1) showed an inhibitory effect on PAA-UV/PAA inactivation, but PAA-UV/PAA was more adaptable in a wide pH range and the presence of humic acid compared to direct UV/PAA. The more severe cell membrane damage and higher reactive oxygen species level in PAA-UV/PAA were evidenced for the first time by flow cytometry. The increased hydroxyl radical generation and higher synergism were primarily responsible for inactivation improvement. This study enhances the further understanding of the PAA-UV/PAA process, and the findings are expected to promote the development of PAA as a promising disinfectant for effective fungi control.