Abstract
The objectives of this study were to determine the efficacy of metabolites of a Streptomyces strain AS1 on (a) spore germination, (b) mycelial growth, (c) control of mycotoxins produced by Penicillium verrucosum (ochratoxin A, OTA), Fusarium verticillioides (fumonisins, FUMs) and Aspergillus fumigatus (gliotoxin) and (d) identify the predominant metabolites involved in control. Initial screening showed that the Streptomyces AS1 strain was able to inhibit the mycelial growth of the three species at a distance, due to the release of secondary metabolites. A macroscopic screening system showed that the overall Index of Dominance against all three toxigenic fungi was inhibition at a distance. Subsequent studies showed that the metabolite mixture from the Streptomyces AS1 strain was very effective at inhibiting conidial germination of P. verrucosum, but less so against conidia of A. fumigatus and F. verticillioides. The efficacy was confirmed in studies on a conducive semi-solid YES medium in BioScreen C assays. Using the BioScreen C and the criteria of Time to Detection (TTD) at an OD = 0.1 showed good efficacy against P. verrucosum when treated with the Streptomyces AS1 extract at 0.95 and 0.99 water activity (aw) when compared to the other two species tested, indicating good efficacy. The effective dose for 50% control of growth (ED50) at 0.95 and 0.99 aw were approx. 0.005 ng/ml and 0.15 μg/ml, respectively, with the minimum inhibitory concentration (MIC) at both aw levels requiring > 40 μg/ml. In addition, OTA production was completely inhibited by 2.5 μg/ml AS1 extract at both aw levels in the in vitro assays. Ten metabolites were identified with four of these being predominant in concentrations > 2 μg/g dry weight biomass. These were identified as valinomycin, cyclo(L-Pro-L-Tyr), cyclo(L-Pro-L-Val) and brevianamide F.
Highlights
The objectives of this study were to determine the efficacy of the Streptomyces AS1 metabolites on (a) spore germination and mycelial growth of P. verrucosum, Fusarium verticillioides and A. fumigatus, (b) efficacy for control of the production of ochratoxin A (OTA), FUMs and GLI and (c) to identify the major metabolites produced by the Streptomyces AS1 strain responsible for the control achieved
The inhibition was best against P. verrucosum (90%) followed by A. fumigatus strain (Mi538; 59%) and F. verticillioides (51%)
The total index of dominance (ID), which was the sum of the individual scores, was 15:0 (AS1:mycotoxigenic strain)
Summary
Present address: Science and Food Technology Research Centre, Malaysian Agricultural and Research Institute, 43400 Serdang, Selangor, Malaysia. Food and feeds such as cereals, nuts and spices are commonly contaminated with mycotoxigenic fungi and mycotoxins that are a serious problem faced by many countries (Lee and Ryu 2017). This causes significant losses to producer countries when their exports are rejected because they do not Mycotoxin Res (2020) 36:225–234 meet the legislative limits, especially in Europe. In a 4-year survey by Limay-Rios et al (2017) of stored wheat in Canada, Penicillium verrucosum was commonly isolated, as well as contamination with OTA, ochratoxin B (OTB) and citrinin
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