Influence of ecophysiological factors on growth, lag phase and ochratoxin A production by Aspergillus niger aggregate strains in irradiated corn grains

AnAzf1 acts as a positive regulator of ochratoxin A biosynthesis in Aspergillus niger

Aspergillus niger produces genotoxic and carcinogenic ochratoxin A (OTA) that severely threatens human and animal health. Transcription factor Azf1 is essential in regulating fungal cell development and primary metabolism. However, its effect and mechanism on secondary metabolism are unclear. Here, we characterized and deleted a Azf1 homolog gene, An15g00120 (AnAzf1), in A. niger, which completely blocked OTA production, and repressed the OTA cluster genes, p450, nrps, hal, and bzip at the transcriptional level. The results indicated that AnAzf1 was a positive regulator of OTA biosynthesis. Transcriptome sequencing results showed that the AnAzf1 deletion significantly upregulated antioxidant genes and downregulated oxidative phosphorylation genes. Enzymes involved in reactive oxygen species (ROS) scavenging, including catalase (CAT) and peroxidase (POD) were increased, and the corresponding ROS levels were decreased. Upregulation of genes (cat, catA, hog1, and gfd) in the MAPK pathway and downregulation of genes in iron homeostasis were associated with decreased ROS levels, linking the altered MAPK pathway and iron homeostasis to lower ROS levels caused by AnAzf1 deletion. Additionally, enzymes including complex I (NADH-ubiquinone oxidoreductase), and complex V (ATP synthase), as well as ATP levels, were significantly decreased, indicating impaired oxidative phosphorylation caused by the AnAzf1-deletion. During lower ROS levels and impaired oxidative phosphorylation, OTA was not produced in ∆AnAzf1. Together, these results strongly suggested that AnAzf1 deletion blocked OTA production in A. niger by a synergistic interference of ROS accumulation and oxidative phosphorylation. KEY POINTS: • AnAzf1 positively regulated OTA biosynthesis in A. niger. • Deletion of AnAzf1 decreased ROS levels and impaired oxidative phosphorylation. • An altered MAPK pathway and iron homeostasis were associated with lower ROS levels.

Influence of a glyphosate-based herbicide on growth parameters and aflatoxin B1 production by Aspergillus section Flavi on maize grains

Glyphosate-based herbicides (GBH) are the main pesticides applied worldwide on maize production. Glyphosate-resistant weeds led to the repeated application of high doses of the pesticide. In addition to environmental conditions, the presence of GBH affects the development of Aspergillus species and aflatoxin B1 (AFB1) production under in vitro conditions. The aim of this work was to evaluate the influence of a commercial GBH on growth and AFB1 production by Aspergillus flavus and Aspergillus parasiticus strains under different water activity (aW) conditions. The following concentrations of active ingredient glyphosate were evaluated: 20, 50, 200 and 500mM. The lag phase prior to growth and growth rate did not change at 20 and 50mM (that is, at field recommended doses) at 0.98 and 0.95 aW; however, at increasing GBH concentrations, between 200 and 500mM, the growth rate decreased at all aW conditions. In general, as the GBH concentration increased, AFB1 production decreased. However, a significant increase in toxin accumulation was found only at one of the aW conditions (0.95) at 21 days with 50mM of GBH in A. flavus and 20 and 50mM of GBH in A. parasiticus. These results show that, even though Aspergillus section Flavi growth did not increase, AFB1 production increased on maize grains at GBH concentrations similar to those of field recommended doses under favorable water availability and temperature conditions.

Impact of some environmental factors on growth and ochratoxin A production by Aspergillus niger and Aspergillus welwitschiae

Ochratoxin A (OTA) is a nephrotoxic mycotoxin which may contaminate various foods and feed products worldwide. Aspergillus niger is one of the species responsible for OTA contamination in grapes and derived products. This species has recently been split into A. niger and Aspergillus welwitschiae. Both species can not be distinguished by phenotypic or extrolite profiles and to date there is no ecophysiological information of A. welwitschiae. The aim of this study was to determine the effects of water activity (a_w) (0.90; 0.95 and 0.98-0.99), culture media (Yeast Extract Sucrose Broth (YESB); Synthetic Grape Juice Medium (SGM); White grape juice (WGJ)) and temperature (15 °C, 25 °C and 35 °C) on the growth and OTA production of four strains of A. niger and six strains of A. welwitschiae. The assay was performed in microtiter plates, determining the absorbance at 530 nm and the concentration of OTA at 1, 2, 4 and 10 days. No significant differences were observed in absorbance and OTA values between the two species under study. The highest absorbance values were recorded in YESB, followed by SGM and WGJ. Absorbance values increased with increasing a_w and temperature. The highest OTA values were obtained at 0.98-0.99 a_w and the best culture media for OTA production was YESB, followed by WGJ and SGM. The studied strains of A. niger produced the highest mean OTA level at 25 °C whereas A. welwitschiae strains produced the highest mean OTA concentration at 15 °C, although not differing significantly from concentration produced at 25 °C. To our knowledge, this is the first report on the impact of some environmental factors on growth and OTA production by A. welwitschiae.

A quantitative study on growth variability and production of ochratoxin A and its derivatives by A. carbonarius and A. niger in grape-based medium

Aspergillus carbonarius and Aspergillus niger are the main responsible fungi for the accumulation of ochratoxin A (OTA) in wine grapes. Some strains are able to convert the parent mycotoxin into other compounds by means of hydrolysis and/or conjugation reactions through their defense mechanisms and enzymatic activity, leading to the formation of a modified mycotoxin. Thus, the variability of growth and metabolite production are inherent to the strain, occurring distinctively even when submitted to similar conditions. In this sense, this contribution aimed at determining the variability in multiplication and production of OTA by strains of A. carbonarius and A. niger isolated from grapes, as well as investigating the formation of modified mycotoxins. Strains were incubated in grape-based medium, and the diameter of the colonies measured daily. The determination of OTA was performed by high-performance liquid chromatography and the identification of modified mycotoxins was carried out using high-resolution mass spectrometry. Variabilities in terms of growth and OTA production were assessed across five different strains. Peak production of OTA was detected on day 15, and a decline on day 21 was observed, indicating that the observed reduction may be associated with the degradation or modification of the OTA over time by the fungus. Ethylamide ochratoxin A, a modified mycotoxin identified in this study, provides evidence that there may be underreporting of total mycotoxin levels in food, increasing uncertainty concerning health risks to the population.

Antifungal and antimycotoxigenic effect of Lactobacillus plantarum CRL 778 at different water activity values

Ochratoxin A (OTA) is a mycotoxin produced by filamentous fungi with high impact in food safety due to its toxicity. In the last decade, the presence of OTA was widely reported in different foods. In this study, the ability of Lactobacillus (L.) plantarum CRL 778 to control growth and OTA production by Aspergillus (A.) niger 13D strain, at different water activity (a_w) values (0.955, 0.964, 0.971, 0.982, and 0.995) was determined in vitro. Both parameters were significantly (p<0.05) reduced by the lactobacilli and the effect depended on a_w. Greatest growth rate inhibition (46.9%) was obtained at a_w=0.995, which is the most suitable value for growth and production of antifungal metabolites (lactic acid, acetic acid, phenyllactic and hydroxyl-phenyllactic acids) by L. plantarum CRL 778. Besides, morphological changes and inhibition of melanin synthesis were observed in colonies of A. niger 13D in presence of L. plantarum CRL 778 at a_w ranged between 0.971 and 0.995. In addition, maximum reduction (90%) of OTA production took place at a_w=0.971, while inhibition of fungi growth was more evident at a_w=0.995. These findings suggest that L. plantarum CRL 778 could be used for control of ochratoxigenic fungal growth and OTA contamination in different fermented foods with a_w values between 0.971 and 0.995.

Influence of temperature, water activity and incubation time on fungal growth and production of ochratoxin A and zearalenone by toxigenic Aspergillus tubingensis and Fusarium incarnatum isolates in sorghum seeds

The major objective of this study was to describe the effect of water activity and temperature on radial growth and production of ochratoxin A (OTA) and zearalenone (ZEA) on sorghum grains of three Aspergillus tubingensis and three Fusarium incarnatum isolates. The water activity range was 0.91-0.99 a_w for F. incarnatum isolates and 0.88-0.99 a_w for A. tubingensis isolates. Temperatures of incubation were 15, 25 and 37°C for both species. Mycotoxin production was determined after 7, 14, 21 and 28days depending on the growth rate of the six isolates. Maximum growth rates (mm/day) were observed at 37°C and 0.99 a_w for A. tubingensis isolates and at 0.99 a_w and 25°C for F. incarnatum isolates. A. tubingensis was able to grow at 15°C only at the highest a_w levels (0.97 and 0.99 a_w). However, at this temperature F. incarnatum grew at 0.94 a_w. Optimum ochratoxin A production was observed at 0.97 a_w×37°C whereas optimal conditions for ZEA production varied from one isolate to another. Moreover, isolates of F. incarnatum from Tunisia do not require high a_w and temperature levels to yield maximum levels of ZEA. In general, our results showed that there is no correlation between the growth and production of ZEA in the case of F. incarnatum. This is the first study on the water activity and temperature effect on growth rate and ZEA production of F. incarnatum. Our results show that sorghum grains not only support growth but also OTA and ZEA production by A. tubingensis and F. incarnatum, respectively.

Effect of post-harvest treatments on the occurrence of ochratoxin A in raw cocoa beans

Cocoa beans are the principal raw material for chocolate manufacture. Moulds have an important place in the change in the quality of cocoa beans due to their role in the production of free fatty acids and mycotoxins, namely ochratoxin A (OTA). This study investigated the impact of the key post-harvest treatments, namely the fermentation and drying methods on OTA contamination of raw cocoa beans. Analytical methods for OTA detection were based on solid-liquid extraction, clean-up using an immunoaffinity column, and identification by reversed-phase HPLC with fluorescence detection. Of a total of 104 randomly selected cocoa samples analysed, 32% had OTA contents above 2 µg kg(-1). Cocoa sourced from pods in a bad state of health had a maximum OTA content of 39.2 µg kg(-1), while that obtained from healthy pods recorded 11.2 µg kg(-1). The production of OTA in cocoa beans increased according to the pod-opening delay and reached 39.2 µg kg(-1) after an opening delay of 7 days after harvest, while 6.1 and 11.2 µg kg(-1) were observed when pods were opened after 0 and 4 days. OTA production also seemed to depend considerably to the cocoa fermentation materials. When using plastic boxes for bean fermentation, the OTA production was enhanced and reached an average OTA content of about 4.9 µg kg(-1), while the raw cocoa treated in banana leaves and wooden boxes recorded 1.6 and 2.2 µg kg(-1) on average respectively. In parallel, the OTA production was not really influenced by either the mixing or the duration of the fermentation or the drying materials.

Effect of water activity and temperature on growth of Aspergillus carbonarius and Aspergillus tubingensis and their interactions on ochratoxin A production

Aspergillus section Nigri are described as the main source of ochratoxin A (OTA) contamination in grapes and wine worldwide. The grape-growing area in Argentina has a wide latitudinal extension with ecological variations that allow the classification of well-demarcated regions. The aims of this study were: to determine the effect of eco-physiological parameters on growth of Aspergillus tubingensis and Aspergillus carbonarius and to evaluate the interaction between these species on OTA production in synthetic grape juice medium under different water activity (aw) and temperature conditions. The results showed that optimal growth conditions for A. tubingensis and A. carbonarius were 0.980 aw and 28 °C, however A. tubingensis grew faster than A. carbonarius at all temperatures and aw tested. OTA production by A. carbonarius was favoured at 20 °C and 0.950-0.965 aw, during 14 days of incubation. The effect of A. tubingensis on OTA production by A. carbonarius was mainly dependent on temperature. At 35 °C, A. tubingensis reduced the OTA production while this effect was not observed at 20 °C. More OTA could be produced as a defence reaction against fungal competitors to maintain niche colonisation, but in this study no effects by a related species were observed.

Influence of light on growth, conidiation and the mutual regulation of fumonisin B2 and ochratoxin A biosynthesis by Aspergillus niger

Aspergillus niger is a fungus able to produce the carcinogenic mycotoxins ochratoxin A (OTA) and fumonisins. We analysed the influence of light of various wavelengths on growth, conidiation, fumonisin B2 (FB2) and OTA biosynthesis by A. niger ITEM 7097. Light from both sides of the spectrum, from long (627 nm) to short wavelengths (470-455 nm), had a stimulating effect on growth, with the highest stimulation under blue (455 nm, 1,700 Lux) and short-wave blue light (390 nm). Conidiation was reduced by 40% under a short blue wavelength (455 nm, 200 Lux), but strongly promoted under light at an even shorter wavelength (390 nm), with an increase of about 200 fold in comparison to the dark. Production of FB2 and OTA was mutually regulated by light. FB2 production was promoted under light conditions: red and blue light in particular increased FB2 biosynthesis by 40%. Conversely, OTA production was greatly inhibited under red and blue light in comparison to dark incubation, with a mean reduction of about 40 fold, indicating a reverse regulation of both biosynthetic pathways. Incubation under a 390 nm wavelength repressed the production of both toxins to non-detectable levels.

The effects of different ecophysiological factors on ochratoxin A production

Mycotoxin biosynthesis is affected by peculiar agro-ecosystem dependent conditions. Accordingly, the correlation between mycotoxin contamination and environmental condition change scenarios should be carefully considered. This is because fungal colonization and mycotoxin contamination could critically impair the availability of food products, especially in poor countries. At any time, the agricultural products might become susceptible to fungal contamination and mycotoxin production in uncontrolled storage. From 2000 to 2010, many studies were carried out in the case of the effects of different parameters on ochratoxin A (OTA) production by fungi in order to avoid providing the appropriate conditions for producing OTA. Water activity (a(w)), temperature, type of species and the substrates were introduced as the most important in these articles. This paper reviews the major published articles regarding the different ecophysiological factors influencing on OTA production.

Impact of non-selective fungicides on the growth and production of ochratoxin A by Aspergillus ochraceus and A. carbonarius in barley-based medium

The aim of this study was to assess the influence of the non-selective fungicides mancozeb, copper oxychloride, and sulfur on the growth and capability for producing ochratoxin A (OTA) of ochratoxigenic isolates of Aspergillus carbonarius and A. ochraceus in barley-based medium. Lag phases and growth rates were determined for each fungicide at different doses, at 15°C and 25°C and at 0.97 a(w). Mancozeb at 40 mg l(-1 )inhibited fungal growth and provided lag phases >24 days at 10-20 mg l(-1) and 15°C. OTA was observed only at 25°C and doses <10 mg l(-1). At 15°C, copper oxychloride proved inhibitory at 800 mg l(-1), while at 25°C growth was not delayed and only high doses decreased OTA levels. Sulfur was inhibitory or provided large lag phases at 5-8 g l(-1) (at 15°C) while at 25°C growth took place even at 8 g l(-1), although OTA levels were low or undetectable. The antifungal activity decreased in the order mancozeb > copper oxychloride > sulfur, and was lower at 25°C than at 15°C. OTA accumulation was affected by the type of fungicide, dose, temperature and time. The efficacy of these fungicides on the growth of A. carbonarius and A. ochraceus and OTA production in barley-based medium is assessed for the first time.

Ecophysiology of Aspergillus section nigri species potential ochratoxin a producers

After aflatoxins, ochratoxin A (OTA) is the most studied mycotoxin due to the toxicological significance in human and animal diets. OTA presence has been extensively reported worldwide in the last decade in several agricultural products. The main OTA producer in tropical and temperate climates is Aspergillus carbonarius followed by species belonging to A. niger aggregate. Currently, many scientists worldwide have studied the influence of water activity and temperature for growth and biosynthesis of OTA by these species on synthetic media. This article reviews ecophysiological studies of Aspergillus section Nigri strains on synthetic media and natural substrates. The results of these investigations suggest that significant amounts of OTA can be produced in only five days and that the use of different storage practices, such as a(W) and temperature levels below 0.930 and 15 °C, respectively, allow controlling fungal contamination and minimizing the OTA production in several products as peanuts, corn, dried grapes and derived products for human consumption.

Aspergillus carbonarius growth and ochratoxin A production on irradiated dried grapes under different water activity and temperature conditions

Grapes have different destinations. The most important in the national and international market is wine production, but another is dehydration to produce raisins. Dried vine fruits are at greater risk of ochratoxin A (OTA) contamination than wine grapes because the ratio of Aspergillus carbonarius to Aspergillus niger aggregate increases during drying. The growth of these species, and consequently OTA production, can be influenced by different environmental factors, the two most important being water activity (aW) and temperature. The objective of the present work was to evaluate the lag phase, growth rate and OTA production by two A. carbonarius isolates on irradiated dried grapes at different aW (0.910, 0.928, 0.955, 0.973 and 0.995), temperatures (15, 25 and 30 °C) and incubation times (7, 14 and 21 days). Growth was observed at all aW and temperature ranges assayed. No significant differences between the growth rates reached at 25 °C and 30 °C by both isolates were observed. At the assayed conditions, OTA production occurred over the whole range of temperatures (15-30 °C), with the maximum at 25 and 30 °C depending on the aW. In general, OTA concentration increased as aW increased with no statistically significant differences at the tested incubation times. This work provides information that can be used by companies for the purpose of preventing A. carbonarius and OTA contamination during storage on this and other substrates (e.g. dried prunes, figs and apricots) destined for human consumption.

Growth and ochratoxin A production by Aspergillus niger group strains in coffee beans in relation to environmental factors

The effect of water activity (aW), temperature and their interactions on lag phase, mycelial growth rate and ochratoxin A (OTA) production at 7, 14 and 21 days of incubation of two OTA-producer strains belonging to Aspergillus niger group on irradiated coffee beans was determined. Irradiated coffee beans were re-hydrated to 0.910-0.995 of aW with sterile distilled water. The temperatures assayed were 15, 25 and 30 °C. Growth assessment was measured every day during the incubation period to calculate the growth rate. OTA production was examined at 7, 14 and 21 days by high-performance liquid chromatography. Optimal aW for growth was 0.995 at 25 °C for RCC4 and RCC20 strains, being 1.10 and 2.36 mm/day, respectively. OTA concentration varied considerably depending on aW, temperature and incubation time assayed. Maximum OTA production was obtained at 0.973 and 0.995 aW at 30 °C for both strains. The results of the present work indicate that knowledge of the optimal and marginal conditions of black Aspergillus growth and OTA production allow methods to be established for preventing the development of these fungal and mycotoxin production on coffee beans. The data obtained provide useful information for predicting the risk factors for OTA contamination on coffee beans.