Comparative study of growth responses and screening of inter-specific OTA production kinetics by A. carbonarius isolated from grapes

Predictive growth kinetic parameters and modelled probabilities of deoxynivalenol production by Fusarium graminearum on wheat during simulated storing conditions

AIMS

Mathematical models were employed to predict the growth kinetic parameters of F. graminearum and the accumulation of deoxynivalenol (DON) during wheat storage as a function of different moisture contents (MCs) and temperatures.

METHODS AND RESULTS

The colony counting method was used to quantify F. graminearum growth under different environmental conditions, and kinetic and probability models were developed to describe the effect of different MCs and temperatures on fungal growth and DON production during wheat storage. Among the employed secondary models (Arrhenius-Davey, Gibson, and Cardinal), the general polynomial best predicted the fungal growth rate under varying temperature and MC during wheat storage. According to the logistic model, DON contamination was correctly predicted in 96.5% of cases.

CONCLUSIONS

The maximum growth rate of fungi was 0.4889±0.092 Log CFU g^-1 d^-1 at 25°C and 30% moisture according to the polynomial model. At below 17°C and ≤15% moisture, no fungal growth was observed. The probability model of toxin production showed no toxin production at less than 15% moisture (a_w ≤ 0.76) and below 15°C.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first application of a probability model of DON production during wheat storage, providing a reference for preventing fungal growth and mycotoxin accumulation by F. graminearum during wheat storage and guaranteeing food product safety.

Evaluation of the stability and antibacterial activity of crude extracts of hydro-endophytic fungi

The production and screening of secondary metabolites of four hydro-endophytes isolated from lotus, and the stability of bioactive compounds was evaluated. Surface-sterilized technique was used to isolate the endophytic fungi (EF) on potato dextrose agar and identified by using morphological and molecular techniques. The extracts were tested for anti-microbial activity against methicillin-resistant Staphylococcus aureus (MRSA) DMST20651, Streptococcus mutans (SM) DMST18777, Staphylococcus epidermidis (SE) ATCC12228, Pseudomonas aeruginosa (PA) TISTR1467, and Propionibacterium acnes (PN) DMST14916. The bacteriostatic and bactericidal activities were determined. Finally, thermal and ultraviolet (UV) stability was evaluated. Four endophyte isolates (EF 14, EF36, EF53, EF58, and EF60) produced secondary metabolites and showed activity against MRSA, SM, SE, PA, and PN, respectively. The crude ethyl acetate (EtOAc) and methanol (MeOH) extract of EF14 showed activity against MRSA with the inhibition zone of 9.00 ± 0.00 and 7.50 ± 0.50 mm, and minimum inhibitory concentration was 4.80 and 4.90 mg/mL, respectively. The minimum bactericidal concentration was 9.60 mg/mL. Whereas, the crude EtOAc and MeOH extract EF60, which were extracted by EtOAc and MeOH, showed inhibition zone of SE as 12.33 ± 0.57 and 12.33 ± 0.57 mm, respectively. Crude EtOAC extracts of EF14 showed highest thermal stability at 55°C-121°C, and UV stability with MRSA and SE, respectively. The results showed that the EtOAc extracts of EF could be potential antibacterial pathogens and displayed UV-thermal stability. This information is beneficial for future investigations, since some bioactive compounds have potential as anti-resistant strains of some bacterial pathogens.

Pest Management and Ochratoxin A Contamination in Grapes: A Review

Ochratoxin A (OTA) is the most toxic member of ochratoxins, a group of toxic secondary metabolites produced by fungi. The most relevant species involved in OTA production in grapes is Aspergillus carbonarius. Berry infection by A. carbonarius is enhanced by damage to the skin caused by abiotic and biotic factors. Insect pests play a major role in European vineyards, and Lepidopteran species such as the European grapevine moth Lobesia botrana are undoubtedly crucial. New scenarios are also emerging due to the introduction and spread of allochthonous pests as well as climate change. Such pests may be involved in the dissemination of OTA producing fungi even if confirmation is still lacking and further studies are needed. An OTA predicting model is available, but it should be integrated with models aimed at forecasting L. botrana phenology and demography in order to improve model reliability.

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.

Control of Aspergillus carbonarius in grape berries by Lactobacillus plantarum: A phenotypic and gene transcription study

The in vitro and in situ antifungal activity of Lactobacillus plantarum against the ochratoxigenic fungus Aspergillus carbonarius was investigated in this study. Four different fungal isolates from grape berries were co-cultured with four different strains of L. plantarum on Malt Extract Agar (MEA) plates at 30 °C. Bacterial strains inhibited fungal growth up to 88% and significantly reduced toxin production up to 100%. In addition, L. plantarum was evaluated as biocontrol agent against A. carbonarius growth and OTA production on table grapes. Temporal studies of bacterial antagonism were performed with two different grape cultivars. Artificially wounded and unwounded berries were pre-treated with 10⁸ CFU/mL bacteria and inoculated with 10⁶ spores/mL of A. carbonarius ochratoxigenic isolates. Biocontrol agents displayed high rate of colonization on grapes during 5 days of incubation at 30 °C. Scanning electron microscopy (SEM) also determined the presence of microorganisms on grape surface. Bacterial strains were effective in controlling fungal infection reaching up to 71% inhibition rates. However the presence of wounds on grape skin facilitated infection of berries by A. carbonarius, since unwounded berries showed lower levels of infection. Results also revealed significant reduction in mycotoxin production ranging between 32% and 92%. Transcriptome analysis following exposure to co-cultivation, exhibited differential expression for each gene studied of AcOTAnrps (Aspergillus carbonarius OTA nonribosomal), AcOTApks (Aspergillus carbonarius OTA polyketide synthase) and laeA, emphasizing the significance of strain variability. The genes AcOTAnrps and laeA were most influenced by the presence of L. plantarum. This work is a contribution for the potential biocontrol of toxigenic fungi in table grapes by lactic acid bacteria (LAB). The above findings underline the significance of bacterial strain variability on the effectiveness of biopreservative features of L. plantarum strains.

Monitoring the Temporal Expression of Genes Involved in Ochratoxin A Production of Aspergillus carbonarius under the Influence of Temperature and Water Activity

The objective of this study was to investigate the effect of environmental factors, namely temperature and water activity, on genes involved in the regulation of ochratoxin A (OTA) production over time. For this purpose, the previously characterized toxigenic Aspergilluscarbonarius Ac29 isolate from Greek vineyards and the A. carbonarius ITEM 5010 reference strain were subjected to combined temperature and water activity (a_w) treatments to study OTA production and relative gene expression. The fungal isolates were grown on a synthetic grape juice liquid medium (SGM) under different temperature (20 °C, 25 °C and 30 °C) and a_w (0.94 and 0.98) regimes. The expression of the AcOTApks, AcOTAnrps, and laeA OTA related genes was investigated using real time PCR. Gene expression was monitored at the same time points, along with fungal biomass and OTA accumulation at three, six and nine days of incubation. In gene expression analysis, stimulation of the biosynthetic genes was observed a few days before any toxin could be detected. This fact may underline a possible early indicator of potential toxin contamination of grapes. However, the transcript levels varied with respect to the different combinations of ecophysiological conditions and time, highlighting a complex regulation of OTA related gene expression of A. carbonarius in the specific medium.