The Influence of NaCl and Glucose Content on Growth and Ochratoxin A Production by Aspergillus ochraceus, Aspergillus carbonarius and Penicillium nordicum

The AwHog1 Transcription Factor Influences the Osmotic Stress Response, Mycelium Growth, OTA Production, and Pathogenicity in Aspergillus westerdijkiae fc-1

Aspergillus westerdijkiae, known as the major ochratoxin A (OTA) producer, usually occurs on agricultural crops, fruits, and dry-cured meats. Microorganisms produce OTA to adapt to the high osmotic pressure environment that is generated during food processing and storage. To investigate the relationship between OTA biosynthesis and the high osmolarity glycerol (HOG) pathway, the transcription factor AwHog1 gene in A. westerdijkiae was functionally characterised by means of a loss-of-function mutant. Our findings demonstrated that the growth and OTA production of a mutant lacking AwHog1 decreased significantly and was more sensitive to high osmotic media. The ΔAwHog1 mutant displayed a lower growth rate and a 73.16% reduction in OTA production in the wheat medium compared to the wild type. After three days of culture, the growth rate of the ΔAwHog1 mutant in medium with 60 g/L NaCl and 150 g/L glucose was slowed down 19.57% and 13.21%, respectively. Additionally, the expression of OTA biosynthesis genes was significantly reduced by the deletion of the AwHog1 gene. The infection ability of the ΔAwHog1 mutant was decreased, and the scab diameter of the pear was 6% smaller than that of the wild type. These data revealed that transcription factor AwHog1 plays a key role in the osmotic response, growth, OTA production, and pathogenicity in A. westerdijkiae.

Seasonal and Geographical Impact on the Mycotoxigenicity of Aspergillus and Fusarium Species Isolated from Smallholder Dairy Cattle Feeds and Feedstuffs in Free State and Limpopo Provinces of South Africa

This study evaluated the impact of seasonal and geographical variations on the toxigenicity of Aspergillus and Fusarium strains previously isolated from smallholder dairy cattle feeds and feedstuffs sampled during summer and winter in the Free State and Limpopo provinces of South Africa (SA). In total, 112 potential toxigenic fungal species were obtained and determined for their capability to produce mycotoxins on solid Czapek Yeast Extract Agar (CYA); followed by liquid chromatography-mass spectrometry (LC-MS/MS) analysis. Our result revealed that 41.96% of the fungal species produced their respective mycotoxins, including aflatoxin B₁ (AFB₁), aflatoxin B₂ (AFB₂), and zearalenone (ZEN), with higher levels of AFB₁ (0.22 to 1045.80 µg/kg) and AFB₂ (0.11 to 3.44 µg/kg) produced by fungal species isolated from summer samples than those in winter [(0.69 to 14.44 µg/kg) and (0.21 to 2.26 µg/kg), respectively]. The same pattern was also observed for AFB₁ and AFB₂ in Limpopo (0.43 to 1045.80 µg/kg and 0.13 to 3.44 µg/kg) and Free State (0.22 to 576.14 µg/kg and 0.11 to 2.82 µg/kg), respectively. More so, ZEN concentrations in summer (7.75 to 97.18 µg/kg) were higher than in winter (5.20 to 15.90 µg/kg). A similar observation was also noted for ZEN in Limpopo (7.80 to 97.18 µg/kg) and Free State (5.20 to 15.90 µg/kg). These findings were confirmed via Welch and Brown-Forsythe tests with significantly (p ≤ 0.05) higher mycotoxin levels produced by fungal strains obtained in samples during summer than those in winter. In contrast, the concentrations of mycotoxins produced by the fungal species from both provinces were not significantly (p > 0.05) different.

Impact of environmental factors on ochratoxin A: From natural occurrence to control strategy

Ochratoxin A (OTA) contamination and the associated issues of food security, food safety and economic loss are widespread throughout the world. The occurrence of OTA depends on ochratoxigenic fungi, foodstuffs and their environment. In this review, natural occurrence and control strategy of OTA, with a focus on the impact of environmental factors, are summarized. First, this manuscript introduces potentially contaminated foodstuffs, including the emerging ones which are not regulated in international legislation. Secondly, it gives an update of native producers based on foodstuffs and OTA biosynthesis. Thirdly, complicated environmental regulation is disassembled into individual factors in order to clarify their regulatory effect and mechanism. Finally, to emphasize control OTA at all stages of foodstuffs from farm to table, strategies used at crop planting, harvest, storage and processing stages are discussed.

Mutual effects on mycotoxin production during co-culture of ochratoxigenic and aflatoxigenic Aspergillus strains

Mycotoxin co-occurrence compromises the safety of food crops worldwide. Environmental factors, as well as fungal interaction, can substantially influence the infectivity of mycotoxigenic fungi and their subsequent production of multi-mycotoxin. Here, we investigated the mutual effects of the co-culture of ochratoxigenic and aflatoxigenic Aspergillus strains on the co-production of ochratoxin A (OTA) and aflatoxin B1 (AFB1). Single cultures of ochratoxigenic A. carbonarius and A. alliaceus grew optimally at 25 °C, whereas aflatoxigenic A. flavus grew optimally at 35 °C. The maximum levels of OTA and AFB1 were achieved at 25 °C, whereas mycotoxin production decreased at 35 °C. During competitive growth of the ochratoxigenic and aflatoxigenic isolates, inhibition or stimulation of mycotoxin production was dependent on the fungal strain, temperature, and the ratio of the spore concentration. Aspergillus carbonarius and A. alliaceus generally produced OTA, with similar patterns of relative OTA levels at all temperatures. AFB1 production by A. flavus in the presence of ochratoxigenic Aspergillus species was inhibited at 25 °C and stimulated at 35 °C. These results indicated that the temperature, presence of other mycotoxigenic Aspergillus species, and ratio of the initial spore concentration significantly contributed to the co-production of OTA and AFB1.

Production of Aflatoxin B1 by Aspergillus parasiticus Grown on a Novel Meat-Based Media

The aim of the present study was to develop meat-based media with compositions similar to those of dry-fermented meat products and to evaluate their use in studying the growth of Aspergillus parasiticus and the kinetics of aflatoxin B1 (AFB1) production. In our previous experiments, we found that the strain A. parasiticus ŽMJ7 produced a high amount of AFB1. Cooked meat agar (CMA2) was used as a novel complex meat-based medium with four variations: CMA2G (CMA2 supplemented with 1% glucose), CMA2YE (CMA2 supplemented with 0.2% yeast extract), and CMA2GYE (CMA2 supplemented with 1% glucose and 0.2% yeast extract). Media were inoculated with an A. parasiticus spore suspension (105 spores/mL) and incubated at 25 °C for up to 15 days. The A. parasiticus lag phase lasted less than 1 day, irrespective of the growth medium, with the exception of control medium CMA1 (cooked meat agar) as an already known meat-based medium. The highest mean colony growth rate was observed on CMA2 and CMA2G. Reversed-phase UPLC-MS/MS analysis was performed to determine the AFB1 concentration in combination with solid phase extraction (SPE). The highest AFB1 concentration in meat-based media was detected in CMA2GYE after 15 days of incubation (13,502 ± 2367 ng/mL media). The results showed that for studying AFB1 production in dry-fermented meat products, novel suitable media such as CMA2-based media are required. This finding could represent a potential concern with regard to the production of dry-fermented meat products.

Optimization of l-malic acid production from acetate with Aspergillus oryzae DSM 1863 using a pH-coupled feeding strategy

Background

Malic acid, a dicarboxylic acid mainly used in the food industry, is currently produced from fossil resources. The utilization of low-cost substrates derived from biomass could render microbial processes economic. Such feedstocks, like lignocellulosic hydrolysates or condensates of fast pyrolysis, can contain high concentrations of acetic acid. Acetate is a suitable substrate for l-malic acid production with the filamentous fungus Aspergillus oryzae DSM 1863, but concentrations obtained so far are low. An advantage of this carbon source is that it can be used for pH control and simultaneous substrate supply in the form of acetic acid. In this study, we therefore aimed to enhance l-malate production from acetate with A. oryzae by applying a pH-coupled feeding strategy.

Results

In 2.5-L bioreactor fermentations, several feeding strategies were evaluated. Using a pH-coupled feed consisting of 10 M acetic acid, the malic acid concentration was increased about 5.3-fold compared to the batch process without pH control, resulting in a maximum titer of 29.53 ± 1.82 g/L after 264 h. However, it was not possible to keep both the pH and the substrate concentration constant during this fermentation. By using 10 M acetic acid set to a pH of 4.5, or with the repeated addition of NaOH, the substrate concentration could be maintained within a constant range, but these strategies did not prove beneficial as lower maximum titers and yields were obtained. Since cessation of malic acid production was observed in later fermentation stages despite carbon availability, a possible product inhibition was evaluated in shake flask cultivations. In these experiments, malate and succinate, which is a major by-product during malic acid production, were added at concentrations of up to 50 g/L, and it was found that A. oryzae is capable of organic acid production even at high product concentrations.

Conclusions

This study demonstrates that a suitable feeding strategy is necessary for efficient malic acid production from acetate. It illustrates the potential of acetate as carbon source for microbial production of the organic acid and provides useful insights which can serve as basis for further optimization.

Modulation of Virulence-Associated Traits in Aspergillus fumigatus by BET Inhibitor JQ1

Aspergillus fumigatus is a disease-causing, opportunistic fungus that can establish infection due to its capacity to respond to a wide range of environmental conditions. Secreted proteins and metabolites, which play a critical role in fungal–host interactions and pathogenesis, are modulated by epigenetic players, such as bromodomain and extraterminal domain (BET) proteins. In this study, we evaluated the in vitro and in vivo capability of the BET inhibitor JQ1 to modulate the extracellular proteins and virulence of A. fumigatus. The abundance of 25 of the 76 extracellular proteins identified through LC-MS/MS proteomic analysis changed following JQ1 treatment. Among them, a ribonuclease, a chitinase, and a superoxide dismutase were dramatically downregulated. Moreover, the proteomic analysis of A. fumigatus intracellular proteins indicated that Abr2, an intracellular laccase involved in the last step of melanin synthesis, was absent in the JQ1-treated group. To investigate at which level this downregulation occurred and considering the ability of JQ1 to modulate gene expression we checked the level of ABR2, Chitinase, and Superoxide dismutase mRNA expression by qRT-PCR. Finally, the capacity of JQ1 to reduce the virulence of A. fumigatus has been proved using Galleria mellonella larvae, which are an in vivo model to evaluate fungal virulence. Overall, the promising activity exhibited by JQ1 suggests that A. fumigatus is sensitive to BET inhibition and BET proteins may be a viable target for developing antifungal agents.

Ochratoxin A in Dry-Cured Ham: OTA-Producing Fungi, Prevalence, Detection Methods, and Biocontrol Strategies-A Review

Traditional dry-cured hams are easily contaminated by toxigenic fungi during the fermentation and ripening stages. The detection and positive rates of ochratoxin A (OTA) are the highest among mycotoxins detected in traditional dry-cured hams, indicating that OTA in hams is a potential safety hazard to human health. This review addresses the mycotoxin-producing fungal species, the toxigenic conditions causing OTA contamination worldwide, the prevalence of OTA contamination in dry-cured hams, and the detection methods applied in OTA analysis. Additionally, this study introduces methods to prevent and control OTA in traditional dry-cured hams. The growth of common mycotoxin-producing fungi and the accumulation of mycotoxins in dry-cured ham can be controlled by a microbial starter. This review provides an important theoretical foundation for the research and control of OTA in traditional dry-cured hams.

Investigation of fungal contamination in medicinal and edible Lycii Fructus through DNA metabarcoding

AIMS

Lycii Fructus (LF) is considered as a 'superfood' due to its health benefits and delicious tastes, which has gained popularity worldwide. However, LF is also a proper host for fungal growth due to its abundant nutrients. Fungal contamination seriously affects the quality and safety of LF and poses threats to consumer health.

METHODS AND RESULTS

In this study, a total of 15 LF samples were collected from five provinces in China, and were divided into five groups based on the collection areas. Fungal contamination in LF was investigated by targeting the internal transcribed spacer 2 region using Illumina Miseq PE300 platform, and the differences of fungal community in groups based on collection areas were compared. Results showed that the fungal contamination was detected in all the 15 LF samples. Ascomycota, Dothideomycetes, Pleosporales and Pleosporaceae were dominant at the phylum, class, order and family levels, respectively. At the genus level, Alternaria, Cladosporium and Fusarium were the three dominant genera. In all, 24 fungal species were identified. Among which, two species, namely Penicillium oxalicum and Trichothecium roseum, were potentially toxigenic.

CONCLUSIONS

All 15 LF samples were detected with fungal contamination. The differences of fungal community in LF samples collected from different areas were observed. DNA metabarcoding was demonstrated as an efficient method to monitor the fungal contamination in LF.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work comprehensively reveals the fungal diversity and composition in LF and provides early warning for potential mycotoxin contamination.

Valorization of a Pyrolytic Aqueous Condensate and Its Main Components for L-Malic Acid Production with Aspergillus oryzae DSM 1863

Pyrolytic aqueous condensate (PAC) might serve as a cost-effective substrate for microbial malic acid production, as it is an unused side stream of the fast pyrolysis of lignocellulosic biomass that contains acetol and acetate as potential carbon sources. In the present study, shake flask cultures were performed to evaluate the suitability of acetol and its combination with acetate as substrates for growth and L-malate production with the filamentous fungus Aspergillus oryzae. Acetol concentrations of up to 40 g/L were shown to be utilized for fungal growth. In combination with acetate, co-metabolization of both substrates for biomass and malate formation was observed, although the maximum tolerated acetol concentration decreased to 20 g/L. Furthermore, malate production on PAC detoxified by a combination of rotary evaporation, overliming and activated carbon treatment was studied. In shake flasks, cultivation using 100% PAC resulted in the production of 3.37 ± 0.61 g/L malate, which was considerably improved by pH adjustment up to 9.77 ± 0.55 g/L. A successful scale-up to 0.5-L bioreactors was conducted, achieving comparable yields and productivities to the shake flask cultures. Accordingly, fungal malate production using PAC was successfully demonstrated, paving the way for a bio-based production of the acid.

Acetate as substrate for L-malic acid production with Aspergillus oryzae DSM 1863

BACKGROUND

Microbial malic acid production is currently not able to compete economically with well-established chemical processes using fossil resources. The utilization of inexpensive biomass-based substrates containing acetate could decrease production costs and promote the development of microbial processes. Acetate is a by-product in lignocellulosic hydrolysates and fast pyrolysis products or can be synthesized by acetogens during syngas fermentation. For the fermentation of these substrates, a robust microorganism with a high tolerance for biomass-derived inhibitors is required. Aspergillus oryzae is a suitable candidate due to its high tolerance and broad substrate spectrum. To pave the path towards microbial malic acid production, the potential of acetate as a carbon source for A. oryzae is evaluated in this study.

RESULTS

A broad acetate concentration range was tested both for growth and malic acid production with A. oryzae. Dry biomass concentration was highest for acetic acid concentrations of 40-55 g/L reaching values of about 1.1 g/L within 48 h. Morphological changes were observed depending on the acetate concentration, yielding a pellet-like morphology with low and a filamentous structure with high substrate concentrations. For malic acid production, 45 g/L acetic acid was ideal, resulting in a product concentration of 8.44 ± 0.42 g/L after 192 h. The addition of 5-15 g/L glucose to acetate medium proved beneficial by lowering the time point of maximum productivity and increasing malic acid yield. The side product spectrum of cultures with acetate, glucose, and cultures containing both substrates was compared, showing differences especially in the amount of oxalic, succinic, and citric acid produced. Furthermore, the presence of CaCO3, a pH regulator used for malate production with glucose, was found to be crucial also for malic acid production with acetate.

CONCLUSIONS

This study evaluates relevant aspects of malic acid production with A. oryzae using acetate as carbon source and demonstrates that it is a suitable substrate for biomass formation and acid synthesis. The insights provided here will be useful to further microbial malic acid production using renewable substrates.

Quantitative Proteomic Profiling of Fungal Growth, Development, and Ochratoxin A Production in Aspergillus ochraceus on High- and Low-NaCl Cultures

Dry-cured meat products are worldwide food with high-salt content, and filamentous fungi are beneficial to the maturation process. However, some salt-tolerant strains of Aspergillus and Penicillium produce ochratoxin A (OTA) on these products and thus threaten food safety. In our study, proteomic analysis was performed to reveal the mechanism of adaptability to high-salt environment by Aspergillus ochraceus. Twenty g/L and 70 g/L NaCl substrates were used to provide medium- and high-NaCl content environments, respectively. The NaCl addition could induce fungal growth, but only 20 g/L NaCl addition could induce spore production while 70 g/L repressed it. Proteomics analysis identified 2646 proteins in A. ochraceus fc-1, of which 237 and 251 were differentially expressed with 20 g/L and 70 g/L NaCl addition, respectively. Potential factors affecting fungal growth and development were identified by GO and KEGG analyses of biological process, cellular component, and molecular function terms. The results revealed that ergosterol synthesis pathway was significantly upregulated with 20 g/L and 70 g/L NaCl addition. However, fungal growth and development including OTA production were complex processes associated with many factors including nutrient uptake, cell membrane integrity, cell cycle, energy metabolism, intracellular redox homeostasis, protein synthesis and processing, autophagy, and secondary metabolism. Reactive oxygen species may be an important window to understand the mechanism that medium-salt content was conducive to intracellular signal transduction while high-salt content caused oxidative stress. The findings would help to improve the processes and storage conditions of dry-cured meat products.