Daniel Richard-Molard

Growth of Fusarium moniliforme and its biosynthesis of fumonisin B1 on maize grain as a function of different water activities

The production of fumonisin by Fusarium moniliforme during its growth on maize depends on extrinsic factors. In particular, experiments on maize grain at different water activities (aw)(1, 0.95, 0.90, 0.85) have demonstrated the influence of aw on fumonisin biosynthesis, and on fungal growth defined by measurement of ergosterol levels. Fumonisin levels dropped threefold when aw was lowered by 5%, but growth rate was unchanged. A 10% reduction in aw from 1 to 0.90 resulted in a 20‐fold drop in fungal growth, and fumonisin production was reduced 300‐fold. At a threshold aw of 0.85–0.86, F. moniliforme exhibited virtually no measurable metabolic activity, and hence no fumonisin production.

Mould growth and conidiation in cereal grains as affected by water activity and temperature

The effects of different water activities (aw) and temperatures on growth of storage moulds (Aspergillus candidus and Penicillium implicatum) on maize and paddy rice grains were measured using ergosterol content and dilution plating. The results showed that the mode of fungal growth depends on aw. In all cases, conidiation occurred more readily at low aw, whereas mycelial growth is more marked at higher aw. The xerotolerant, thermophilic characteristics of A. candidus were more clearly revealed by the ergosterol content, which reflected mycelial development, than by enumeration of spores formed. The ergosterol content proved to be a better index of technological quality of cereals (assessed by acidity) than the number of fungal propagules. A 10‐fold increase in the number of spores corresponded to a doubling of the ergosterol content.

Study of the biosynthesis of fumonisins B1, B2 and B3 by different strains of Fusarium moniliforme.

The relationship between fungal growth and the production of fumonisin on maize grain by 25 strains of Fusarium moniliforme of different origins has been investigated. Although sporulation was essentially the same for all the strains (about 108 propagules g−1 dry matter), ergosterol assays revealed marked variations in fungal biomass. All strains studied produced highly variable amounts of fumonisin B1, the highest levels being observed in strains of ergosterol content above 400 μg g−1. However, no correlation could be established between the synthesized biomass and the quantity of fumonisins produced. We verified that ergosterol is an indicator of mycelial growth, and therefore of the potential toxicity of the analysed grain.

Fatty acid composition and molecular order of phospholipids from Eurotium chevalieri in response to changes in water activity

Summary: The mycelial growth of Eurotium chevalieri was examined at different water activities (a w) using glycerol as the osmoticum. Growth was optimal at 0.90 a w and restricted at 0.995 a w highlighting the xerophilic behaviour of E. chevalieri. Decreased a w produced an increase in the proportion of oleic acid (C18:1) at the expense of the proportion of linoleic acid (C18:2) of cellular phospholipids. The degree of unsaturation of phospholipid fatty acids showed a 20 % decrease between 0.995 and 0.80 a w of growth. Steady-state fluorescence anisotropy (r s) and fluorescence lifetime (τ) measurements for liposomes prepared from cellular phospholipids of E. chevalieri and labelled with DPH (1,6-diphenyl-1,3,5-hexatriene) were made at 25 °C. The lipid order parameter (S, describing molecular order) and the rotational correlation time (ϕ, describing molecular dynamics) were calculated from r s and τ data. Except at 0.995 a w, a decrease in a w was accompanied by increasing r s and S values, indicating a rigidification of membranes, while ϕ values were not significantly different. Plots of order parameters and their first derivatives as a function of temperature exhibited break areas in the temperature range 20-48 °G. These large temperature ranges for lipid transitions could correspond to chain melting of complex lipid systems which made up the liposomes prepared from phospholipids of E. chevalieri. However, as a w decreased, the transition temperatures increased globally, between 0.97 and 0.90 a w.

Labeled proteinase inhibitors: Versatile tools for the characterization of serine proteinases in solid-phase assays

Peptidyl chloromethyl ketones were used for the specific labeling of proteinases by attaching a biotin group to the N-terminal end of the peptide. Such labeled peptide inhibitors allowed the detection and quantitation of proteolytic enzymes immobilized on the plastic surface of a microtiter plate, as well as on nitrocellulose. The validity of these solid-phase assays was demonstrated using subtilisin Carlsberg as a model enzyme and biotinyl-epsilon-aminocaproyl-L-alanyl-L-alanyl-L-propyl-L-phenylal++ + anyl- chloromethyl ketone as a specific reagent. In addition to being usable for the screening of a particular proteinase in a large number of samples, these assays can be adapted for the analysis of specific proteolytic enzyme present in complex mixtures.

Degradation of Maize Stored at Reduced Water Activities by Eurotium chevalieri in Relation to Lipid Content

Abstract Cultures of a xerotolerant mold, Eurotium chevalieri , were grown on full-fat (FM) and defatted (DM) sterilized maize millings stored at reduced water activities (a w ) of 0.80 and 0.74 at 30°C. The mold growth was estimated by colony forming unit (CFU) plate-count method and by ergosterol determination. The growth rates measured by the CFU method showed no difference between FM and DM at reduced a w . However, ergosterol contents were markedly higher in FM which indicated the formation of a higher fungal biomass in media containing lipids. Fungal lipase activity was detected and was shown to be responsible for hydrolysis of lipids to free fatty acids, directly assimilated by E. chevalieri . Under conditions of maize storage at reduced a w , fatty acids released from hydrolysis of maize triglycerides could be the sole available carbon source for fungal growth.