Screening Methods for Isolation of Biocontrol Epiphytic Yeasts against Penicillium digitatum in Lemons

Enhancing biological control of postharvest green mold in lemons: Synergistic efficacy of native yeasts with diverse mechanisms of action

Argentina is among the most important lemon fruit producers in the world. Penicillium digitatum is the primary lemon fungal phytopathogen, causing green mold during the postharvest. Several alternatives to the use of synthetic fungicides have been developed, being the use of biocontrol yeasts one of the most promising. Although many of the reports are based on the use of a single yeast species, it has been shown that the combination of agents with different mechanisms of action can increase control efficiency through synergistic effects. The combined use of native yeasts with different mechanisms of action had not been studied as a biological control strategy in lemons. In this work, the mechanisms of action of native yeasts (Clavispora lusitaniae AgL21, Clavispora lusitaniae AgL2 and Clavispora lusitaniae AcL2) with biocontrol activity against P. digitatum were evaluated. Isolate AgL21 was selected for its ability to form biofilm, colonize lemon wounds, and inhibit fungal spore germination. The compatibility of C. lusitaniae AgL21 with two killer yeasts of the species Kazachstania exigua (AcL4 and AcL8) was evaluated. In vivo assays were then carried out with the yeasts applied individually or mixed in equal cell concentrations. AgL21 alone was able to control green mold with 87.5% efficiency, while individual killer yeasts were significantly less efficient (43.3% and 38.3%, respectively). Inhibitory effects were increased when C. lusitaniae AgL21 and K. exigua strains were jointly applied. The most efficient treatment was the combination of AgL21 and AcL4, reaching 100% efficiency in wound protection. The combination of AgL21 with AcL8 was as well promising, with an efficiency of 97.5%. The combined application of native yeasts showed a synergistic effect considering that the multiple mechanisms of action involved could hinder the development of green mold in lemon more efficiently than using single yeasts. Therefore, this work demonstrates that the integration of native yeasts with diverse modes of action can provide new insights to formulate effective microbial consortia. This could lead to the development of tailor-made biofungicides, allowing control of postharvest fungal diseases in lemons while remaining competitive with traditionally used synthetic chemicals.

Identification of antagonistic yeasts as potential biocontrol agents: Diverse criteria and strategies

Plant pathogenic and food spoilage microorganisms cause serious losses in crop production and severe damage during food manufacturing, transportation and storage. Synthetic antimicrobial agents are commonly used to control their propagation and harmful activities. However, the recent trend is shifting from chemicals towards safer and more eco-friendly alternatives. The use of antagonistic microorganisms as biological antimicrobial agents is becoming popular throughout the world to replace chemical agents. High numbers of microorganisms have turned out to exert adverse/inhibitory effects on other microorganisms including pathogens and spoiling strains. However, most of them are only active under laboratory conditions and their activity is sensitive to environmental changes. Only a small number of them can be used to manufacture biological protective products on an industrial scale. Therefore, there is a great need to identify additional antagonists. Yeasts have come to the forefront of attention because antimicrobial antagonism is fairly widespread among them. In the recent years, numerous excellent review articles covered various aspects of the phenomenon of antimicrobial antagonism of yeasts. However, none of them dealt with how antagonistic yeasts can be sought and identified, despite the high number and diverse efficiency of screening and identification procedures. As researchers working in different laboratories use different criteria and different experimental set-ups, a yeast strain found antagonistic in one laboratory may prove to be non-antagonistic in another laboratory. This review aims to provide a comprehensive and partially critical overview of the wide diversity of identification criteria and procedures to help researchers choose appropriate screening and identification strategies.

Vishniacozyma victoriae: An endophytic antagonist yeast of kiwifruit with biocontrol effect to Botrytis cinerea

Botrytis cinerea is a pathogenic fungus to fruit, biocontrol is a promising approach to relieve this issue. In this study, Vishniacozyma victoriae is an endophytic yeast extracted from kiwifruit, was used to enhance the resistance of host to B. cinerea. The results showed that lesion diameter of the kiwifruit inoculated with B. cinerea was 55.16 %, 50.57 %, and 48.07 % lower than that of inoculated with V. victoriae + B. cinerea on 4th, 8th, and 12th day, respectively. On 12th day, the total organic acid content and energy charge of kiwifruit inoculated with B. cinerea were 19.25 % and 7.95 % lower than those inoculated with V. victoriae + B. cinerea. These indicated that V. victoriae used the organic acids and energy of host to colonize in the wound, which prevented B. cinerea from contacting the host. Accordingly, V. victoriae is a promising biocontrol yeast to inhibit the infection of B. cinerea on kiwifruit.

The GRAS Salts of Na2SiO3 and EDTA-Na2 Control Citrus Postharvest Pathogens by Disrupting the Cell Membrane

Sodium silicate (Na₂SiO₃) and ethylenediaminetetraacetic acid disodium salt (EDTA-Na₂) are inorganic salts classified as 'Generally Recognized as Safe' (GRAS) compounds with great advantages in controlling various pathogens of postharvest fruits and vegetables. Here, we determined the median effective concentration (EC₅₀) of Na₂SiO₃ (0.06%, 0.05%, 0.07% and 0.08%) and EDTA-Na₂ (0.11%, 0.08%, 0.5%, and 0.07%) against common pathogens affecting postharvest citrus fruit, including Penicillium digitatum, Penicillium italicum, Geotrichum citri-aurantii, and Colletotrichum gloeosporioides. Na₂SiO₃ and EDTA-Na₂ treatments at the EC₅₀ decreased the spore germination rate, visibly disrupted the spore cell membrane integrity, and significantly increased the lipid droplets (LDs) of the four postharvest pathogens. Moreover, both treatments at EC₅₀ significantly reduced the disease incidence of P. italicum (by 60% and 93.335, respectively) and G. citri-aurantii (by 50% and 76.67%, respectively) relative to the control. Furthermore, Na₂SiO₃ and EDTA-Na₂ treatment resulted in dramatically lower disease severity of the four pathogens, while also demonstrating no significant change in citrus fruit quality compared with the control. Therefore, Na₂SiO₃ and EDTA-Na₂ present a promising approach to control the postharvest diseases of citrus fruit.

Environmental reservoirs of the drug-resistant pathogenic yeast Candida auris

Candia auris is an emerging human pathogenic yeast; yet, despite phenotypic attributes and genomic evidence suggesting that it probably emerged from a natural reservoir, we know nothing about the environmental phase of its life cycle and the transmission pathways associated with it. The thermotolerant characteristics of C. auris have been hypothesised to be an environmental adaptation to increasing temperatures due to global warming (which may have facilitated its ability to tolerate the mammalian thermal barrier that is considered a protective strategy for humans against colonisation by environmental fungi with pathogenic potential). Thus, C. auris may be the first human pathogenic fungus to have emerged as a result of climate change. In addition, the release of antifungal chemicals, such as azoles, into the environment (from both pharmaceutical and agricultural sources) is likely to be responsible for the environmental enrichment of resistant strains of C. auris; however, the survival and dissemination of C. auris in the natural environment is poorly understood. In this paper, we critically review the possible pathways through which C. auris can be introduced into the environment and evaluate the environmental characteristics that can influence its persistence and transmission in natural environments. Identifying potential environmental niches and reservoirs of C. auris and understanding its emergence against a backdrop of climate change and environmental pollution will be crucial for the development of effective epidemiological and environmental management responses.

Biocontrol potential of wine yeasts against four grape phytopathogenic fungi disclosed by time-course monitoring of inhibitory activities

Grapes' infection by phytopathogenic fungi may often lead to rot and impair the quality and safety of the final product. Due to the concerns associated with the extensive use of chemicals to control these fungi, including their toxicity for environment and human health, bio-based products are being highly preferred, as eco-friendlier and safer alternatives. Specifically, yeasts have shown to possess antagonistic activity against fungi, being promising for the formulation of new biocontrol products.In this work 397 wine yeasts, isolated from Portuguese wine regions, were studied for their biocontrol potential against common grapes phytopathogenic fungal genera: Aspergillus, Botrytis, Mucor and Penicillium. This set comprised strains affiliated to 32 species distributed among 20 genera. Time-course monitoring of mold growth was performed to assess the inhibitory activity resulting from either diffusible or volatile compounds produced by each yeast strain. All yeasts displayed antagonistic activity against at least one of the mold targets. Mucor was the most affected being strongly inhibited by 68% of the tested strains, followed by Botrytis (20%), Aspergillus (19%) and Penicillium (7%). More notably, the approach used allowed the detection of a wide array of yeast-induced mold response profiles encompassing, besides the decrease of mold growth, the inhibition or delay of spore germination and the complete arrest of mycelial extension, and even its stimulation at different phases. Each factor considered (taxonomic affiliation, mode of action and fungal target) as well as their interactions significantly affected the antagonistic activity of the yeast isolates. The highest inhibitions were mediated by volatile compounds. Total inhibition of Penicillium was achieved by a strain of Metschnikowia pulcherrima, while the best performing yeasts against Mucor, Aspergillus and Botrytis, belong to Lachancea thermotolerans, Hanseniaspora uvarum and Starmerella bacillaris, respectively. Notwithstanding the wide diversity of yeasts tested, only three strains were found to possess a broad spectrum of antagonistic activity, displaying strong or very strong inhibition against the four fungal targets tested. Our results confirm the potential of wine yeasts as biocontrol agents, while highlighting the need for the establishment of fit-for-purpose selection programs depending on the mold target, the timing, and the mode of application.

Microbial Intervention: An Approach to Combat the Postharvest Pathogens of Fruits

Plants host diverse microbial communities, which undergo a complex interaction with each other. Plant-associated microbial communities provide various benefits to the host directly or indirectly, viz. nutrient acquisition, protection from pathogen invaders, mitigation from different biotic and abiotic stress. Presently, plant-associated microbial strains are frequently utilized as biofertilizers, biostimulants and biocontrol agents in greenhouse and field conditions and have shown satisfactory results. Nowadays, the plant/fruit microbiome has been employed to control postharvest pathogens and postharvest decay, and to maintain the quality or shelf life of fruits. In this context, the intervention of the natural fruit microbiome or the creation of synthetic microbial communities to modulate the functional attributes of the natural microbiome is an emerging aspect. In this regard, we discuss the community behavior of microbes in natural conditions and how the microbiome intervention plays a crucial role in the postharvest management of fruits.

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.

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.