Killer Yeasts for the Biological Control of Postharvest Fungal Crop Diseases

Amphibian skin bacteria display antifungal activity and induce plant defense mechanisms against Botrytis cinerea

Botrytis cinerea is the causal agent of gray mold, which affects a wide variety of plant species. Chemical agents have been used to prevent the disease caused by this pathogenic fungus. However, their toxicity and reduced efficacy have encouraged the development of new biological control alternatives. Recent studies have shown that bacteria isolated from amphibian skin display antifungal activity against plant pathogens. However, the mechanisms by which these bacteria act to reduce the effects of B. cinerea are still unclear. From a diverse collection of amphibian skin bacteria, three proved effective in inhibiting the development of B. cinerea under in vitro conditions. Additionally, the individual application of each bacterium on the model plant Arabidopsis thaliana, Solanum lycopersicum and post-harvest blueberries significantly reduced the disease caused by B. cinerea. To understand the effect of bacteria on the host plant, we analyzed the transcriptomic profile of A. thaliana in the presence of the bacterium C32I and the fungus B. cinerea, revealing transcriptional regulation of defense-related hormonal pathways. Our study shows that bacteria from the amphibian skin can counteract the activity of B. cinerea by regulating the plant transcriptional responses.

Recent Advances in the Yeast Killer Systems Research

Biocidic phenotype is common in yeast strains isolated from a variety of natural and industrial habitats [...].

Tryptophan enhances biocontrol efficacy of Metschnikowia citriensis FL01 against postharvest fungal diseases of citrus fruit by increasing pulcherriminic acid production

The aim of this work was to study the capability and mechanism of enhancing the yield of pulcherriminic acid (PA) produced by Metschnikowia citriensis FL01 with the help of tryptophan for the control of postharvest diseases on citrus caused by Penicillium italicum, Geotrichum citri-aurantii and Penicillium digitatum. The adding of 10 mmol/L tryptophan to the growth medium resulted in the widest pulcherrimin pigment zone produced by M. citriensis FL01. The adding of tryptophan to the growth medium upregulated A3136 and A3022 gene expression (responsible for leucyl-tRNA biosynthesis from leucine), downregulated A1350 gene expression (responsible for the biosynthesis of leucine to branched-chain fatty acids), and decreased the content of intracellular leucine in M. citriensis FL01, speculating that the addition of tryptophan in the growth medium induced leucine conversion toward leucyl-tRNA in M. citriensis FL01. Moreover, the adding of tryptophan to the growth medium upregulated PULs (responsible PA biosynthesis) and Snf2 (transcriptional regulator) gene expression and promoted intracellular, extracellular or total PA production by M. citriensis FL01 in liquid medium. In addition, the addition of tryptophan in the growth medium showed no effect on the growth of M. citriensis FL01 itself in liquid medium, while the population dynamics in citrus fruit wounds of M. citriensis FL01 with the addition of tryptophan in the growth medium were increased compared with those of M. citriensis FL01. What's more, M. citriensis FL01 with the addition of tryptophan in the growth medium completely inhibited the growth of pathogens in vitro. The disease incidences and lesion diameters of blue mold, sour rot and green mold on citrus fruit were lower in group which treated with M. citriensis FL01 containing tryptophan in the growth medium than that treated with M. citriensis FL01 alone. Overall, the postharvest biocontrol of citrus with M. citriensis FL01 containing 10 mmol/L tryptophan in the growth medium is a promising approach to protect these fruits from blue mold, sour rot and green mold.

Emerging Postharvest Technologies to Enhance the Shelf-Life of Fruit and Vegetables: An Overview

Quality losses in fresh produce throughout the postharvest phase are often due to the inappropriate use of preservation technologies. In the last few decades, besides the traditional approaches, advanced postharvest physical and chemical treatments (active packaging, dipping, vacuum impregnation, conventional heating, pulsed electric field, high hydrostatic pressure, and cold plasma) and biocontrol techniques have been implemented to preserve the nutritional value and safety of fresh produce. The application of these methodologies after harvesting is useful when addressing quality loss due to the long duration when transporting products to distant markets. Among the emerging technologies and contactless and non-destructive techniques for quality monitoring (image analysis, electronic noses, and near-infrared spectroscopy) present numerous advantages over the traditional, destructive methods. The present review paper has grouped original studies within the topic of advanced postharvest technologies, to preserve quality and reduce losses and waste in fresh produce. Moreover, the effectiveness and advantages of some contactless and non-destructive methodologies for monitoring the quality of fruit and vegetables will also be discussed and compared to the traditional methods.

Antifungal activity of non-conventional yeasts against Botrytis cinerea and non-Botrytis grape bunch rot fungi

Grapes harbour a plethora of non-conventional yeast species. Over the past two decades, several of the species have been extensively characterised and their contribution to wine quality is better understood. Beyond fermentation, some of the species have been investigated for their potential as alternative biological tools to reduce grape and wine spoilage. However, such studies remain limited to a few genera. This work aimed to evaluate the antagonistic activity of grape must-derived non-conventional yeasts against Botrytis cinerea and non-Botrytis bunch-rotting moulds and to further elucidate mechanisms conferring antifungal activity. A total of 31 yeast strains representing 21 species were screened on different agar media using a dual culture technique and liquid mixed cultures, respectively. Pichia kudriavzevii was the most potent with a minimum inhibitory concentration of 10² cells/mL against B. cinerea but it had a narrow activity spectrum. Twelve of the yeast strains displayed broad antagonistic activity, inhibiting three strains of B. cinerea (B05. 10, IWBT FF1 and IWBT FF2), a strain of Aspergillus niger and Alternaria alternata. Production of chitinases and glucanases in the presence of B. cinerea was a common feature in most of the antagonists. Volatile and non-volatile compounds produced by antagonistic yeast strains in the presence of B. cinerea were analysed and identified using gas and liquid chromatography mass spectrometry, respectively. The volatile compounds identified belonged mainly to higher alcohols, esters, organosulfur compounds and monoterpenes while the non-volatile compounds were cyclic peptides and diketopiperazine. To our knowledge, this is the first report to demonstrate inhibitory effect of the non-volatile compounds produced by various yeast species.

Microbial volatile organic compounds: Antifungal mechanisms, applications, and challenges

The fungal decay of fresh fruits and vegetables annually generates substantial global economic losses. The utilization of conventional synthetic fungicides is damaging to the environment and human health. Recently, the biological control of post-harvest fruit and vegetable diseases via antagonistic microorganisms has become an attractive possible substitution for synthetic fungicides. Numerous studies have confirmed the potential of volatile organic compounds (VOCs) for post-harvest disease management. Moreover, VOC emission is a predominant antifungal mechanism of antagonistic microorganisms. As such, it is of great significance to discuss and explore the antifungal mechanisms of microbial VOCs for commercial application. This review summarizes the main sources of microbial VOCs in the post-harvest treatment and control of fruit and vegetable diseases. Recent advances in the elucidation of antifungal VOC mechanisms are emphasized, and the applications of VOCs produced from antagonistic microorganisms are described. Finally, the current prospects and challenges associated with microbial VOCs are considered.

Application of Bioactive Coatings with Killer Yeasts to Control Post-Harvest Apple Decay Caused by Botrytis cinerea and Penicillium italicum

A new method was proposed to produce alginate bio-films containing Pichia membranifaciens and Wickerhamomyces anomalus killer yeast to control the post-harvest fungal decay in organic apples caused by Botrytis cinerea and Penicillium italicum. Coatings with W. anomalus killer yeast effectively controlled the growth of P. italicum during storage at 22 °C. W. anomalus killer yeast incorporated in alginate reduced the P. italicum incidence from 90% (control) to 35% after 14 days of storage at 22 °C. Alginate biofilms with W. anomalus or P. membranifaciens also limited the incidence of the fungal decay of apples inoculated with B. cinerea compared with the control fruits, although the antagonistic capability against B. cinerea was lower than against P. italicum. The survival of W. anomalus cells in alginate coating was higher than P. membranifaciens. The incorporation of killer yeasts into alginate had no significant effect on the mechanical properties (tensile strength, percent elongation at break) of alginate coating, however, they increased the thickness of the biofilm. The bioactive coating reduced the fruit weight loss and had no significant effects on the fruit firmness during storage at 2 °C. As organic apples, produced without any synthetic fungicides, are especially prone to fungal decay during storage, the proposed alginate biofilms containing killer yeast seem to be a very promising solution by offering non-chemical, biological control of post-harvest pathogens.

Recent advances in research on biocontrol of postharvest fungal decay in apples

Apple is the largest fruit crop produced in temperate regions and is a popular fruit worldwide. It is, however, susceptible to a variety of postharvest fungal pathogens, including Penicillium expansum, Botrytis cinerea, Botryosphaeria dothidea, Monilia spp., and Alternaria spp. Decays resulting from fungal infections severely reduce apple quality and marketable yield. Biological control utilizing bacterial and fungal antagonists is an eco-friendly and effective method of managing postharvest decay in horticultural crops. In the current review, research on the pathogenesis of major decay fungi and isolation of antagonists used to manage postharvest decay in apple is presented. The mode of action of postharvest biocontrol agents (BCAs), including recent molecular and genomic studies, is also discussed. Recent research on the apple microbiome and its relationship to disease management is highlighted, and the use of additives and physical treatments to enhance biocontrol efficacy of BCAs is reviewed. Biological control is a critical component of an integrated management system for the sustainable approaches to apple production. Additional research will be required to explore the feasibility of developing beneficial microbial consortia and novel antimicrobial compounds derived from BCAs for postharvest disease management, as well as genetic approaches, such as the use of CRISPR/Cas9 technology.

Native Cultivable Bacteria from the Blueberry Microbiome as Novel Potential Biocontrol Agents

Blueberry production is affected by fungal postharvest pathogens, including Botrytis cinerea and Alternaria alternata, the causative agents of gray mold disease and Alternaria rot, respectively. Biocontrol agents adapted to blueberries and local environments are not known to date. Here, we report on the search for and the identification of cultivable blueberry epiphytic bacteria with the potential to combat the aforementioned fungi. Native, blueberry-borne bacterial strains were isolated from a plantation in Tucumán, Argentina and classified based on 16S rRNA gene sequences. Antagonistic activities directed at B. cinerea and A. alternata were studied in vitro and in vivo. The 22 bacterial strains obtained could be attributed to eleven different genera: Rosenbergiella, Fictibacillus, Bacillus, Pseudomonas, Microbacterium, Asaia, Acinetobacter, Curtobacterium, Serratia, Sphingomonas and Xylophilus. Three strains displaying antagonistic impacts on the fungal pathogens were identified as Bacillus velezensis (BA3 and BA4) and Asaia spathodeae (BMEF1). These strains are candidates for biological control agents of local blueberry production and might provide a basis for the development of eco-friendly, sustainable alternatives to synthetic pesticides.

Identification and Molecular Characterization of Novel Mycoviruses in Saccharomyces and Non-Saccharomyces Yeasts of Oenological Interest

Wine yeasts can be natural hosts for dsRNA, ssRNA viruses and retrotransposon elements. In this study, high-throughput RNA sequencing combined with bioinformatic analyses unveiled the virome associated to 16 Saccharomyces cerevisiae and 8 non-Saccharomyces strains of oenological interest. Results showed the presence of six viruses and two satellite dsRNAs from four different families, two of which-Partitiviridae and Mitoviridae-were not reported before in yeasts, as well as two ORFan contigs of viral origin. According to phylogenetic analysis, four new putative mycoviruses distributed in Totivirus, Cryspovirus, and Mitovirus genera were identified. The majority of commercial S. cerevisiae strains were confirmed to be the host for helper L-A type totiviruses and satellite M dsRNAs associated with the killer phenotype, both in single and mixed infections with L-BC totiviruses, and two viral sequences belonging to a new cryspovirus putative species discovered here for the first time. Moreover, single infection by a narnavirus 20S-related sequence was also found in one S. cerevisiae strain. Considering the non-Saccharomyces yeasts, Starmerella bacillaris hosted four RNAs of viral origin-two clustering in Totivirus and Mitovirus genera, and two ORFans with putative satellite behavior. This study confirmed the infection of wine yeasts by viruses associated with useful technological characteristics and demonstrated the presence of complex mixed infections with unpredictable biological effects.

Identification of Volatile Organic Compounds in Extremophilic Bacteria and Their Effective Use in Biocontrol of Postharvest Fungal Phytopathogens

Phytopathogenic fungal growth in postharvest fruits and vegetables is responsible for 20-25% of production losses. Volatile organic compounds (VOCs) have been gaining importance in the food industry as a safe and ecofriendly alternative to pesticides for combating these phytopathogenic fungi. In this study, we analysed the ability of some VOCs produced by strains of the genera Bacillus, Peribacillus, Pseudomonas, Psychrobacillus and Staphylococcus to inhibit the growth of Alternaria alternata, Botrytis cinerea, Fusarium oxysporum, Fusarium solani, Monilinia fructicola, Monilinia laxa and Sclerotinia sclerotiorum, in vitro and in vivo. We analysed bacterial VOCs by using GC/MS and 87 volatile compounds were identified, in particular acetoin, acetic acid, 2,3-butanediol, isopentanol, dimethyl disulphide and isopentyl isobutanoate. In vitro growth inhibition assays and in vivo experiments using cherry fruits showed that the best producers of VOCs, Bacillus atrophaeus L193, Bacillus velezensis XT1 and Psychrobacillus vulpis Z8, exhibited the highest antifungal activity against B. cinerea, M. fructicola and M. laxa, which highlights the potential of these strains to control postharvest diseases. Transmission electron microscopy micrographs of bacterial VOC-treated fungi clearly showed antifungal activity which led to an intense degeneration of cellular components of mycelium and cell death.

Possibilities for the Biological Control of Mycotoxins in Food and Feed

Seeking useful biological agents for mycotoxin detoxification has achieved success in the last twenty years thanks to the participation of many multidisciplinary teams. We have recently witnessed discoveries in the fields of bacterial genetics (inclusive of next-generation sequencing), protein encoding, and bioinformatics that have helped to shape the latest perception of how microorganisms/mycotoxins/environmental factors intertwine and interact, so the road is opened for new breakthroughs. Analysis of literature data related to the biological control of mycotoxins indicates the ability of yeast, bacteria, fungi and enzymes to degrade or adsorb mycotoxins, which increases the safety and quality of susceptible crops, animal feed and, ultimately, food of animal origin (milk, meat and eggs) by preventing the presence of residues. Microbial detoxification (transformation and adsorption) is becoming a trustworthy strategy that leaves no or less toxic compounds and contributes to food security. This review summarizes the data and highlights the importance and prospects of these methods.