The role of extrolites secreted by nonaflatoxigenic Aspergillus flavus in biocontrol efficacy.

AIMS: Field control of aflatoxin contamination is thought to occur through competitive exclusion of native aflatoxigenic fungi by introduced nonaflatoxigenic Aspergillus flavus biocontrol strains. In this study, we explored another possible mechanism that could increase the efficacy of biocontrol strains-the production of secreted compounds termed extrolites. METHODS AND RESULTS: Using four Aspergillus strains (one nonaflatoxigenic and three aflatoxigenic) from the same geographic region (Louisiana), we devised experiments whereby each aflatoxigenic strain was grown on media that had first been colonized by the nonaflatoxigenic strain. We observed noticeable reduction in growth and reduced production of aflatoxin and cyclopiazonic acid for all three aflatoxigenic strains when grown in the presence of extrolite secretions from the nonaflatoxigenic strain. CONCLUSIONS: We provide evidence that biocontrol strain extrolites may improve the efficacy of pre and postharvest aflatoxin reduction. SIGNIFICANCE AND IMPACT OF THE STUDY: Our finding, that extrolites secreted by nonaflatoxigenic A. flavus strains potentially abate growth and toxin levels of aflatoxin-producing strains, should allow for us to elucidate the mechanism of how the reduction in toxigenic strains occurs, and potentially identify better biocontrol strains. Identification and isolation of the active extrolites may afford a supplemental method to mitigate aflatoxin production.

Blue light (470 nm) effectively inhibits bacterial and fungal growth.

UNLABELLED: Blue light (470 nm) LED antimicrobial properties were studied alone against bacteria and with or without the food grade photosensitizer, erythrosine (ERY) against filamentous fungi. Leuconostoc mesenteroides (LM), Bacillus atrophaeus (BA) or Pseudomonas aeruginosa (PA) aliquots were exposed on nutrient agar plates to Array 1 (AR1, 0·2 mW cm(-2)) or Array 2 (AR2, 80 mW cm(-2)), which emitted impure or pure blue light (0-300 J cm(-2)), respectively. Inoculated control (room light only) plates were incubated (48 h) and colonies enumerated. The antifungal properties of blue light combined with ERY (11·4 and 22·8 µmol l(-1)) on Penicillium digitatum (PD) and Fusarium graminearum (FG) conidia were determined. Conidial controls consisted of: no light, room light-treated conidia and ERY plus room light. Light-treated (ERY + blue light) conidial samples were exposed only to AR2 (0-100 J cm(-2)), aliquots spread on potato dextrose agar plates, incubated (48 h, 30°C) and colonies counted. Blue light alone significantly reduced bacterial and FG viability. Combined with ERY, it significantly reduced PD viability. Blue light is lethal to bacteria and filamentous fungi although effectiveness is dependent on light purity, energy levels and microbial genus. SIGNIFICANCE AND IMPACT OF THE STUDY: Light from two arrays of different blue LEDs significantly reduced bacterial (Leuconostoc mesenteroides, Bacillus atrophaeus and Pseudomonas aeruginosa) viabilities. Significant in vitro viability loss was observed for the filamentous fungi, Penicillium digitatum and Fusarium graminearum when exposed to pure blue light only plus a photosensitizer. F. graminearum viability was significantly reduced by blue light alone. Results suggest that (i) the amount of significant loss in bacterial viability observed for blue light that is pure or with traces of other wavelengths is genus dependent and (ii) depending on fungal genera, pure blue light is fungicidal with or without a photosensitizer.

Phytochemical glyceollins, isolated from soy, mediate antihormonal effects through estrogen receptor alpha and beta.

The flavonoid family of phytochemicals, particularly those derived from soy, has received attention regarding their estrogenic activity as well as their effects on human health and disease. In addition to these flavonoids other phytochemicals, including phytostilbene, enterolactone, and lignans, possess endocrine activity. The types and amounts of these compounds in soy and other plants are controlled by both constitutive expression and stress-induced biosynthesis. The health benefits of soy-based foods may, therefore, be dependent upon the amounts of the various hormonally active phytochemicals within these foods. The aim was to identify unique soy phytochemicals that had not been previously assessed for estrogenic or antiestrogenic activity. Here we describe increased biosynthesis of the isoflavonoid phytoalexin compounds, glyceollins, in soy plants grown under stressed conditions. In contrast to the observed estrogenic effects of coumestrol, daidzein, and genistein, we observed a marked antiestrogenic effect of glyceollins on ER signaling, which correlated with a comparable suppression of 17 beta-estradiol-induced proliferation in MCF-7 cells. Further evaluation revealed greater antagonism toward ER alpha than ER beta in transiently transfected HEK 293 cells. Competition binding assays revealed a greater affinity of glyceollins for ER alpha vs. ER beta, which correlated to greater suppression of ER alpha signaling with higher concentrations of glyceollins. In conclusion, we describe the phytoalexin compounds known as glyceollins, which exhibit unique antagonistic effects on ER in both HEK 293 and MCF-7 cells. The glyceollins as well as other phytoalexin compounds may represent an important component of the health effects of soy-based foods.

Volatile trans-2-hexenal, a soybean aldehyde, inhibits Aspergillus flavus growth and aflatoxin production in corn.

UNLABELLED: Trans-2-hexenal, a volatile aldehyde, is produced by soybean (Glycine max [L.] Merr) and other plants via the lipoxygenase pathway. In vitro tests showed it significantly (P < 0.001) reduced Aspergillus flavus germinating conidial viability at 10 µM, with approximately 95% viability reduction observed at 20 µM. The viability of nongerminated conidia was not reduced. To test the effectiveness of this volatile to prevent fungal growth in stored corn, trans-2-hexenal was pumped intermittently into glass jars containing corn. Experiments were performed to determine the ability of 2 different pump cycle time-courses to prevent A. flavus growth on sterile corn (23% moisture). Intermittently (30-min pumping period) over 7 d, this volatile was pumped through 350 g of corn kernels inoculated with 1 mL of 3 × 104 conidia of A. flavus. Controls consisted of (1) sterile corn, (2) corn inoculated with A. flavus with no pumped air, and (3) corn inoculated with A. flavus with intermittently pumped air. Aflatoxin B1 (AFB1), viability counts, and aldehyde concentration in the headspace were performed in each experiment. To determine whether an increased time period between volatile pumping would prevent A. flavus growth, a 2nd series of experiments were performed that were similar to the 1st series except that trans-2-hexenal (only) was pumped for a 30-min period every 12 h. Experiments were performed 3 times for each time course. Both experiments showed that intermittent pumping of volatile trans-2-hexenal significantly (P < 0.001) prevented A. flavus growth and aflatoxin B1 production over a 7-d period. PRACTICAL APPLICATION: Results from this study indicate that intermittent pumping of volatile trans-2-hexenal could be used to protect stored corn from A. flavus growth and aflatoxin contamination.

Effect of soybean volatile compounds on Aspergillus flavus growth and aflatoxin production.

Soybean homogenates produced volatile compounds upon exposure to lipase. These induced volatiles were identified by SPME. Seventeen volatile compounds identified by SPME were chosen for determination of their ability to inhibit Aspergillus flavus growth and aflatoxin B(1) (AFB1) production in a solid media assay. These volatiles included aldehydes, alcohols, ketones, and furans. Of the tested compounds, the aldehydes showed the greatest inhibition of fungal growth and AFB1 production. These compounds inhibited up to 100% of the observed growth and AFB1 production as compared to the controls. The greatest activity by the aldehydes to disrupt growth was ranked as follows: 2,4 hexadienal > benzaldehyde > 2-octenal > (E)-2-heptenal > octanal > (E)-2-hexenal > nonanal > hexanal. The greatest activity by the aldehydes to reduce AFB1 was ranked as follows: (E)-2-hexenal > 2,4 hexadienal > (E)-2-heptenal > hexanal > nonanal. (E)-2-hexenal and (E)-2-heptenal were tested further in an A. flavus-inoculated corn kernel assay. Both compounds prevented colonization by A. flavus and eliminated AFB1 production when exposed to compound volumes < 10 muL as also shown in the solid media assay. The results suggest that soybeans react to lipase by production of potent antifungal volatiles.

Effect of biotic elicitors on enrichment of antioxidant properties and induced isoflavones in soybean.

The antioxidant properties of methanolic extracts from soybean obtained with germination, wounding, and application of biotic elicitors were evaluated. Also, the relationship between observed antioxidant properties and compositional changes in isoflavone content was determined. The 2 biotic elicitors used in this study were the food-grade fungus Aspergillus sojae and A. sojae cell wall extract. Isoflavone content was determined by C(18) reverse phase high-performance chromatography coupled with a photodiode array detector. Antioxidant activities of the extracts were measured using 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging and beta-carotene cooxidation in a linoleate system. Higher antioxidant activities were observed in wounded and elicitor-treated extracts when compared with nonwounded control extracts. In addition, the phenolic content was higher in extracts from wounded and elicitor-treated soybean. Germination for 3 d slightly decreased total isoflavone content (-4.3%); however, wounding increased total isoflavone content (25.8%). The soybean extracts from seeds treated with A. sojae biotic elicitors had the highest total isoflavone contents (9.8 to 11.6 mg/g extract) and displayed the highest antioxidant activities in both the DPPH and beta-carotene assays. Also identified in the wounded and elicitor-treated extracts were the induced isoflavones glyceollins that contributed to the higher isoflavone contents observed.