Naresh Magan

Post-harvest fungal ecology: Impact of fungal growth and mycotoxin accumulation in stored grain

Grain quality after harvest is influenced by a wide variety of abiotic and biotic factors and has been studied as a stored grain ecosystem. Important factors include grain and contaminant mould respiration, insects and mites, and the key environmental factors of water availability and temperature. Interactions between these factors influence the dominance of fungi, particularly mycotoxigenic species. Studies have shown that growth, mycotoxin production, competitiveness and niche occupation by mycotoxigenic species are influenced by the presence of other contaminant moulds and environmental factors. This has been demonstrated for both Fusarium culmorum and deoxynivalenol production, Aspergillus ochraceus/Penicillium verruscosum and ochratoxin production and Fusarium section Liseola and fumonisin production. Interactions between mycotoxigenic spoilage fungi and insects do occur but have not been studied thoroughly. Some insects disseminate mycotoxigenic species, others are known to use spoilage moulds as a food source, while others avoid certain fungal species. Thus, a more holistic ecological view is needed when considering management approaches to long-term-safe storage of cereal grains after harvest.

Electronic noses and disease diagnostics

Rapid developments in sensor technology have facilitated the production of devices — known as electronic noses — that can detect and discriminate the production profiles of volatile compounds from microbial infections in situ. Such qualitative and semi-quantitative approaches could have a significant role in the early diagnosis and detection of microbial diseases. Using artificial intelligence and web-based knowledge systems, electronic noses might also have a valuable role in monitoring disease epidemiology.

Volatiles as an indicator of fungal activity and differentiation between species, and the potential use of electronic nose technology for early detection of grain spoilage.

There is significant interest in methods for the early detection of quality changes in cereal grains. The development of electronic nose technology in recent years has stimulated interest in the use of characteristic volatiles and odours as a rapid, early indication of deterioration in grain quality. This review details the current status of this area of research. The range of volatiles produced by spoilage fungi in vitro and on grain are described, and the key volatile groups indicative of spoilage are identified. The relationship between current grain quality descriptors and the general classes of off-odours as defined in the literature, e.g. sour, musty, are not very accurate and the possible correlation between these for wheat, maize and other cereals, and volatiles are detailed. Examples of differentiation of spoilage moulds and between grain types using an electronic nose instrument are described. The potential for rapid and remote grain classification and future prospects for the use of such technology as a major descriptor of quality are discussed.

Mycotoxins in food: detection and control.

Measuring Risks. Risk Assessment and Risk Management of Mycotoxins in Food. Modelling Exposure to Mycotoxins. Current Regulations Governing Mycotoxin Limits in Foods. Sampling for Mycotoxins. New Techniques for Mycotoxin Analysis. Rapid Detection of Mycotoxigenic Fungi in Plants. Controlling Risks. The Use of HACCP Systems to Control Mycotoxins: The Case of Cereals. Environmental Conditions Affecting Mycotoxins. The Control of Mycotoxins during Storage. The Control of Mycotoxins During Secondary Processing. Risk Assessment and Management in Practice: Grapes and Wine. Controlling Mycotoxins in Animal Feed. Particular Mycotoxins. Ochratoxin A. Patulin. Zearalenone. Fumonisins. Other Mycotoxins. Deoxynivalenol. Appendix. Control of Mycotoxins during Raw Material Production.

Water and temperature relations of growth and ochratoxin A production by Aspergillus carbonarius strains from grapes in Europe and Israel

Aims:  This study investigated the in vitro effects of water activity (aw; 0·85–0·987) and temperature (10–40°C) on growth and ochratoxin A (OTA) production by two strains of Aspergillus carbonarius isolated from wine grapes from three different European countries and Israel on a synthetic grape juice medium representative of mid‐veraison (total of eight strains).

Relationship between growth and mycotoxin production by Fusarium species, biocides and environment

Fusarium head blight of cereals has, in recent years, become one of the most important pre-harvest diseases worldwide. This paper examines the in vitro efficacy of fungicides to control Fusarium species in cereals and the efficacy in the field on both Fusarium infection of ripening ears as well as their impact on mycotoxin production. Field studies suggest that fungicides such as tebuconazole and metconazole give good control of both Fusarium infection of ears and control of deoxynivalenol (DON) production. However, azoxystrobin and related fungicides are less effective, and grain from treated crops has sometimes been found to have increased concentrations of DON and nivalenol. Studies of isolates of Fusarium culmorum from different parts of Europe showed that complex interactions occur between environmental factors, fungicide type and isolate in relation to growth inhibition and DON production. These studies confirmed the ineffectiveness of azoxystrobin and suggest that environmental stress factors, particularly water availability and temperature, and low fungicide doses may stimulate mycotoxin production by Fusaria in vitro and in wheat grain.

Manipulation of intracellular glycerol and erythritol enhances germination of conidia at low water availability

The insect pathogens Beauveria bassiana, Metarhizium anisopliae and Paecilomyces farinosus can be effective biocontrol agents when relative humidity (RH) is close to 100%. At reduced water availability, germination of propagules, and therefore host infection, cannot occur. Cultures of B. bassiana, M. anisopliae and P. farinosus were grown under different conditions to obtain conidia with a modified polyol and trehalose content. Conidia with higher intracellular concentrations of glycerol and erythritol germinated both more quickly and at lower water activity (aw) than those from other treatments. In contrast, conidia containing up to 235.7 mg trehalose g-1 germinated significantly (P < 0.05) more slowly than those with an equivalent polyol content but less trehalose, regardless of water availability. Conidia from control treatments did not germinate below 0.951-0.935 aw (identical to 95.1-93.5% RH). In contrast, conidia containing up to 164.6 mg glycerol plus erythritol g-1 germinated down to 0.887 aw (identical to 88.7% RH). These conidia germinated below the water availability at which mycelial growth ceases (0.930-0.920 aw). Germ tube extension rates reflected the percentage germination of conidia, so the most rapid germ tube growth occurred after treatments which produced conidia containing the most glycerol and erythritol. This study shows for the first time that manipulating polyol content can extend the range of water availability over which fungal propagules can germinate. Physiological manipulation of conidia may improve biological control of insect pests in the field.

Environmental factors, in vitro interactions, and niche overlap between Fusarium moniliforme, F. proliferatum, and F. graminearum, Aspergillus and Penicillium species from maize grain

The effects of temperature and water availability on growth and interactions between fumonisin-producing isolates of Fusarium moniliforme and F. proliferatum and seven other fungi from maize grain were determined in vitro . The type of interaction and index dominance ( I d ) between species were markedly influenced by temperature and a w . Generally, F. moniliforme and F. proliferatum were very competitive and dominant against the Penicillium spp. and A. flavus . They were in turn dominated by A. niger , but mutually antagonistic when paired with F. graminearum and A. ochraceus . Under slightly drier conditions ( a w ) A. ochraceus became more competitive and dominant over the fumonisin-producing species. A. flavus was dominant only at 30°C and a w . F. moniliforme and F. proliferatum demonstrated dominance against all species over a range of temperatures and 0.994 to 0.96 a w . At lower a w levels they were less competitive. The growth rate of the two fumonisin-producing species was significantly reduced by F. graminearum , regardless of a w . F. moniliforme and F. proliferatum reduced growth of Penicillium and Aspergillus spp., especially at > 0.96 a w . At a w , growth of these species was unaffected. Using Biolog plates the effect of a w and temperature on utilization patterns of carbon sources in maize were evaluated for the first time. The niche overlap indices relative to F. moniliforme and F. proliferatum were determined and compared with that of each interacting species. NOIs for F. moniliforme and F. proliferatum were > 0.90 at > 0.96 a w and 25 and 30°, indicative of co-existence with other species. Most of species had NOIs > 0.90, except in some cases when paired with F. moniliforme , where NOIs d and NOI methods both suggested that the niche overlap between species was in a state of flux and significantly influenced by both temperature and water availability. This suggests that interpretation of I d NOIs carried out under one set of environmental conditions may be misleading when considering interactions between species and also where screening for biocontrol potential is being considered.

Stress induction of mycotoxin biosynthesis genes by abiotic factors

Systematic expression analysis of mycotoxin biosynthesis genes by real-time PCR and microarray was carried out to examine the relationship between growth and general expression patterns in relation to single environmental factors such as temperature, water activity (a(w)) and pH and water activity x temperature interactions. For single parameters, one major peak of expression occurred close to optimum growth conditions. However, a second minor peak was observed under suboptimal growth conditions, when intermediate environmental stress was imposed on Aspergillus parasiticus (afl genes), Penicillium verrucosum (ota genes) and Fusarium culmorum (tri genes). This expression profile pattern was more pronounced in relation to changes in temperature and a(w) than to pH. In a two-factorial experimental design with temperature xa(w) regimes, again two peaks of expression were observed for cluster genes after microarray analysis, one close to those giving optimal growth and one under imposed stress conditions. Interestingly, when the activity of single genes of the microarray data were plotted in relation to the two parameters, again a two-peak expression profile became obvious independently for both parameters. Expression of the mycotoxin biosynthesis genes was followed exactly by phenotypic mycotoxin production. This expression profile appears to be generic across the mycotoxigenic fungi examined.

Effect of water activity and temperature on growth and fumonisin B1 and B2 production by Fusarium proliferatum and F. moniliforme on maize grain

S. MARIN, V. SANCHIS, I. VINAS, R. CANELA AND N. MAGAN. 1995. The effect of different water activities (aw, 0.968, 0.956, 0.944, 0.925) and temperature (25°C and 30°C) on colonization and production of fumonisin B1 (FB1) and B2 (FB2) on sterile layers of maize by Fusarium proliferatum and F. moniliforme isolates was determined over periods of 6 weeks. Generally, both F. moniliforme and F. proliferatum grew faster with increasing aw and best at 30°C. All three isolates produced more FB1 than FB2 regardless of aw or temperature. Very little FB1 and FB2 were produced at 0.925 aw, with maximum produced at 0.956 and 0.968 aw at both temperatures tested. Most FB1 and FB2 were produced by F. moniliforme (25N), followed by F. proliferatum isolates (73N and 131N). At all aw levels and both temperatures there was an increase in FB1 and FB2 concentration with time. Statistical analyses of aw, temperature, time, two‐ and three‐way interactions showed some significant differences between isolates and FB1 and FB2 production.