The role of oxidative stress in the growth of the indoor mold Cladosporium cladosporioides under water dynamics

Enhancement of soil aggregation and physical properties through fungal amendments under varying moisture conditions

Soil structure and aggregation are crucial for soil functionality, particularly under drought conditions. Saprobic soil fungi, known for their resilience in low moisture conditions, are recognized for their influence on soil aggregate dynamics. In this study, we explored the potential of fungal amendments to enhance soil aggregation and hydrological properties across different moisture regimes. We used a selection of 29 fungal isolates, recovered from soils treated under drought conditions and varying in colony density and growth rate, for single-strain inoculation into sterilized soil microcosms under either low or high moisture (≤-0.96 and -0.03 MPa, respectively). After 8 weeks, we assessed soil aggregate formation and stability, along with soil properties such as soil water content, water hydrophobicity, sorptivity, total fungal biomass and water potential. Our findings indicate that fungal inoculation altered soil hydrological properties and improved soil aggregation, with effects varying based on the fungal strains and soil moisture levels. We found a positive correlation between fungal biomass and enhanced soil aggregate formation and stabilization, achieved by connecting soil particles via hyphae and modifying soil aggregate sorptivity. The improvement in soil water potential was observed only when the initial moisture level was not critical for fungal activity. Overall, our results highlight the potential of using fungal inoculation to improve the structure of agricultural soil under drought conditions, thereby introducing new possibilities for soil management in the context of climate change.

The role of hygrodynamic resistance compared to biofilm formation in helping pathogenic bacteria dominate air-conditioning units recovered from odour problems

We previsouly found that installing filters in odourous air-conditioning units (ACUs) to block the entry of skin squames could well tackle the odour problems. In this study, we revisited and sampled the ACUs installed with filters earlier to study the bacterial communities inside the ACUs using 16S amplicon sequencing. We identified 26 genera and found that the skin bacteria isolated in the previous work were absent in this study. Two pathogenic bacteria, Methylobacterium and Sphingomonas, dominated ACUs instead. Afterwards, these two bacteria were identified to species level (Methylobacterium organophilum and Sphingomonas paucimobilis, respectively), and examined in terms of their biofilm formation ability and resistance to changing moisture conditions together with another prevalent species isolated in our previous study, namely Micrococcus luteus, in order to understand the mechanisms of the survival of bacteria in ACUs. In general, M. organophilum and M. luteus showed good biofilm formation ability at all tested temperature levels, but S. paucimobilis only displayed limited biofilm formation. Whereas, all these three bacteria well maintained their survival after wet-dry cycles. These results suggest that compared to biofilm formation, ability to survive under hygrodynamics tends to play a more important role in helping bacteria dominate ACUs. Further, this study implies that the absence of odour problem does not guarantee a healthy environment, more attentions should be given to limit the abundance of hydrodynamic-resistant pathogenic bacteria.

Mechanisms of indoor mold survival under moisture dynamics, a special water treatment approach within the indoor context

Mold contamination is one of the most important causes for indoor air pollution. Previous studies have indicated the feasibility of employing wet-dry cycles, a special water treatment approach in indoor environments, to control indoor mold contamination. However, the underlying mechanisms regulating the responses of indoor molds to changing moisture conditions remains to be elucidated. Here, we studied the mechanisms regulating the responses to wet-dry cycles (termed as moisture dynamics) in Aspergillus penicillioides, Cladosporium cladosporioides, and Aspergillus niger. First, the dormant spores of each mold species were grown to the swollen stage. Next, swollen spores were incubated at different water activity (a_w) levels (0.4, 0.6 and 0.8 a_w) for up to 15 days. Afterward, the viability, lipid peroxidation and antioxidant activities (both enzymatic and non-enzymatic) of treated molds were determined. Our results show that the mold species that survived better under moisture dynamics also encountered less oxidative damage and exhibited stronger antioxidant activities. Moreover, lower RH imposed severer oxidative stress to C. cladosporioides and A. niger. Pearson correlation coefficient indicate significant correlations between oxidative stress and a_w of dry periods, oxidative damage and mold survival, as well as oxidative responses and mold survival. Collectively, these results imply that oxidative stress adaptation regulates the viability of A. penicillioides, C. cladosporioides, and A. niger in response to moisture dynamics. Our findings facilitate the development of novel engineering solutions for indoor air pollution.

Temperature versus Relative Humidity: Which Is More Important for Indoor Mold Prevention?

Temperature is known as one of the abiotic factors that can affect mold growth. Many mold growth prediction models consider temperature as one of the parameters that can significantly impact mold growth indoors, and hence temperature has been targeted by different indoor mold prevention strategies on different premises. For example, European guidelines for libraries suggest a temperature of 19 °C to preserve books. However, running low temperature air-conditioning (AC) costs substantially more energy, and thus a higher temperature (e.g., 25.5 °C) has been regularly proposed as the recommended indoor temperature for general indoor environments in Hong Kong. It is, therefore, needed to understand whether or not the reduction of indoor temperature would lead to better effectiveness of mold prevention. Using Cladosporium cladosporioides (C. cladosporioides) as the model, its germinating spores were challenged in C. cladosporioides to wet-dry cycles with different combinations of relative humidity (RH, 40%, 60% and 80%) and temperature (19 °C and 28 °C) levels. The survival, lipid peroxidation and catalase (CAT) activity of the treated spores were monitored and compared. C. cladosporioides spores showed similar levels of viability, lipid peroxidation and CAT activity when they were exposed to 19 °C and 28 °C at the same RH, but substantially lower survival and higher oxidative stress were observed under the wet-dry cycles with 40% RH dry periods compared with 60% and 80% RH at both temperatures, suggesting that indoor temperature does not tend to affect the resistance of C. cladosporioides to wet-dry cycles as significantly as the RH level of the dry period. Collectively, this study suggests a more important role for moisture over temperature in indoor mold prevention. The outcome of this study may facilitate the sustainable management of indoor mold problems in buildings.

Light Irradiation Coupled with Exogenous Metal Ions to Enhance Exopolysaccharide Synthesis from Agaricus sinodeliciosus ZJU-TP-08 in Liquid Fermentation

To promote Agaricus sinodeliciosus var. Chaidam ZJU-TP-08 growth and metabolites accumulation, a novel integrated strategy was developed by adopting high levels of metal ions coupled with light treatment. The results revealed that yellow and blue light could significantly promote biomass and exopolysaccharides production, respectively. Furthermore, the yellow–blue light shift strategy could stimulate exopolysaccharides formation. Ca²⁺ ions coupled with blue light mostly promoted exopolysaccharides production related to oxidative stress, which was 42.00% and 58.26% higher than that of Ca²⁺ ions coupled with the non-light and dark cultivation without Ca²⁺ ions in 5-L bioreactor. RNA-seq was performed to uncover the underlined molecular mechanism regulated by light-induced gene expressions in exopolysaccharides biosynthesis and oxidative stress. The findings of this work provide valuable insights into adopting metal ions coupled with the light-assisted method for the macrofungus submerged fermentation for exopolysaccharides production.

Effect of Cladosporium cladosporioides on the Composition of Mycoflora and the Quality Parameters of Table Eggs during Storage

The eggshells of 120 experimental one-day-old table eggs were contaminated with the spore suspension of Cladosporium cladosporioides, divided into three groups (A–C) and stored at three different temperatures (3 °C, 11 °C and 20 °C) for 28 days. Visible growth of molds on/in experimental eggs was not observed within the entire storage period. No significant differences in the numbers of molds were found between particular groups of eggs. However, the composition of egg mycoflora was greatly influenced by storage conditions. Three mold genera were identified using the PCR method. The highest mold numbers were determined on Day 14 (Groups A and C) and Day 21 (Group B) when the maximum relative humidity and dew point temperature were recorded. On the same days, the dominance of Penicillium spp. and the minimum eggshell firmness were observed. Noticeable changes in egg quality were observed in eggs stored at 20 °C, and most of these eggs were downgraded at the end of storage period. The growth ability differed significantly among three mold genera. Penicillium spp. and Fusarium spp. showed better growth intensity at increased values (0.91–0.94) of water activity (aw) indicating a possible risk associated with the occurrence of mycotoxins in the egg contents.

Current challenges for shaping the sustainable and mold-free hygienic indoor environment in humid regions

Indoor mold grows ubiquitously in humid areas and can affect occupants' health. To prevent indoor mold contamination, one of the key measures suggested by the World Health Organisation and United States Environmental Protection Agency is to maintain an indoor relative humidity (RH) level below 75% or at 30-60%, respectively. However, in tropical and subtropical areas, maintaining these suggested RH levels is equivalent to operating a 24-h air-conditioner (AC) or dehumidifier, which is energy-consuming. As a large part of building expense, the operation time of ACs has been regularly proposed to be cut down because of the requirement of building sustainability. This leads to a trade-off between sustainable building performance and indoor mold hygiene. To balance this trade-off, more sustainable alternatives, such as those that target physical environments (e.g. nutrient and temperature level) or apply new surface coating technologies to inhibit mold growth, have been launched. Despite these initiatives, indoor mold contamination remains an unresolved issue, mainly because these alternative measures only exhibit limited effectiveness or require extra effort. This review aims to summarize the currently adopted mold control measures and discuss their limitations as well as the direction for the future development of sustainable mold control strategies. SIGNIFICANCE AND IMPACT OF THE STUDY: People spend most of their time indoors and hence the presence of indoor mold contamination can compromise the occupants' health. With the wake of climate change which is expected to see an increase in RH and temperature, tropical and subtropical areas are even more prone to mold contamination than they used to be. This study may help facilitate the development of sustainable and effective mold control strategies in the indoor environment.