Isolation, identification, and activity in vitro of killer yeasts against Colletotrichum gloeosporioides isolated from tropical fruits

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.

Biocontrol potential of Bacillus velezensis wr8 secondary metabolites against Penicillium sp

The aim of this study was to isolate and identify native Bacillus from citrus fruits, with inhibition phenotypes to Penicillium sp. We investigated the antifungal effect of Bacillus velezensis wr8 on the postharvest pathogens Penicillium sp. inoculated on fruits, as well as on the growth of these fungi on Petri dishes with defined media. MALDI-TOF MS was conducted to enlighten the underlying mechanism. Results showed that Bacillus velezensis wr8 significantly inhibited Penicillium sp. growth in vitro. Moreover, secondary metabolites suppressed the disease development of citrus fruits artificially inoculated with Penicillium sp. in 25 °C. Furthermore, MALDI-TOF MS indicated that lipoprotein with the molecular mass of 30.2 kDa was a key component about against Penicillium sp. In addition, the secondary metabolites with antibacterial activity against Escherichia coli showed antimicrobial peptide with the molecular weight of 9.8 kDa. These results demonstrated that Bacillus velezensis wr8 could produce lipoprotein and antimicrobial peptide to inhibit Penicillium sp. and Escherichia coli which has broad application prospect in the future development. Finally, Bacillus velezensis wr8 is to provide data support for the development and utilization of high activity bacteriocin at room temperature and its application in the field of food safety.

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.

A Sustainable Alternative for Postharvest Disease Management and Phytopathogens Biocontrol in Fruit: Antagonistic Yeasts

Postharvest diseases of fruits caused by phytopathogens cause losses up to 50% of global production. Phytopathogens control is performed with synthetic fungicides, but the application causes environmental contamination problems and human and animal health in addition to generating resistance. Yeasts are antagonist microorganisms that have been used in the last years as biocontrol agents and in sustainable postharvest disease management in fruits. Yeast application for biocontrol of phytopathogens has been an effective action worldwide. This review explores the sustainable use of yeasts in each continent, the main antagonistic mechanisms towards phytopathogens, their relationship with OMIC sciences, and patents at the world level that involve yeast-based-products for their biocontrol.

Killer Yeasts for the Biological Control of Postharvest Fungal Crop Diseases

Every year and all over the world the fungal decay of fresh fruit and vegetables frequently generates substantial economic losses. Synthetic fungicides, traditionally used to efficiently combat the putrefactive agents, emerged, however, as the cause of environmental and human health issues. Given the need to seek for alternatives, several biological approaches were followed, among which those with killer yeasts stand out. Here, after the elaboration of the complex of problems, we explain the hitherto known yeast killer mechanisms and present the implementation of yeasts displaying such phenotype in biocontrol strategies for pre- or postharvest treatments to be aimed at combating postharvest fungal decay in numerous agricultural products.

Brazilian fungal diversity represented by DNA markers generated over 20 years

Molecular techniques using fungal DNA barcoding (ITS) and other markers have been key to identifying the biodiversity of different geographic areas, mainly in megadiverse countries. Here, we provide an overview of the fungal diversity in Brazil based on DNA markers of phylogenetic importance generated since 1996. We retrieved fungal sequences of ITS, LSU, SSU, tef1-α, β-tubulin, rpb1, rpb2, actin, chitin synthase, and ATP6 from GenBank using different field keywords that indicated their origin in Brazil. A total of 19,440 sequences were recovered. ITS is the most representative marker (11,209 sequences), with 70.1% belonging to Ascomycota, 18.6% Basidiomycota, 10.2% unidentified, 1.1% Mucoromycota, two sequences of Olpidium bornovanus (Fungi incertae sedis), one sequence of Blastocladiomycota (Allomyces arbusculus), and one sequence of Chytridiomycota (Batrachochytrium dendrobatidis). Considering the sequences of all selected markers, only the phyla Cryptomycota and Entorrhizomycota were not represented. Based on ITS, using a cutoff of 98%, all sequences comprise 3047 OTUs, with the majority being Ascomycota (2088 OTUs) and Basidiomycota (681 OTUs). Previous numbers based mainly on morphological and bibliographical data revealed 5264 fungal species from Brazil, with a predominance of Basidiomycota (2741 spp.) and Ascomycota (1881 spp.). The unidentified ITS sequences not assigned to a higher taxonomic level represent 1.61% of all ITS sequences sampled and correspond to 38 unknown class-level lineages (75% cutoff). A maximum likelihood phylogeny based on LSU illustrates the fungal classes occurring in Brazil.

Biological Control of Fruit Rot and Anthracnose of Postharvest Mango by Antagonistic Yeasts from Economic Crops Leaves

To select antagonistic yeasts for the control of fruit rot caused by Lasiodiplodiatheobromae and anthracnose caused by Colletotrichum gloeosporioides in postharvest mango fruit, 307 yeast strains isolated from plant leaves were evaluated for their antagonistic activities against these two fungal pathogens in vitro. Torulaspora indica DMKU-RP31, T. indica DMKU-RP35 and Pseudozyma hubeiensis YE-21 were found to inhibit the growth of L. theobromae whereas only Papiliotrema aspenensis DMKU-SP67 inhibited the growth of C. gloeosporioides. Antagonistic mechanisms of these four antagonistic yeasts in vitro consisted of the production of antifungal volatile organic compounds (VOCs), biofilm formation and siderophore production. T. indica DMKU-RP35 was the most effective strain in controlling fruit rot on postharvest mango fruits. Its action was comparable to that of the fungicide, benomyl, reducing the disease severity by 82.4%, whereas benomyl revealed 87.5% reduction. P. aspenensis DMKU-SP67 reduced anthracnose severity by 94.1%, which was comparable to that of using benomyl (93.9%). The antifungal VOCs produced by these yeast strains also reduced the severity of these diseases on postharvest mango fruits but at lower rates than using yeast cells. Therefore, these antagonistic yeasts have the potential for use as biological control agents for the control of fruit rot and anthracnose diseases.

The ecology of killer yeasts: Interference competition in natural habitats

Killer yeasts are ubiquitous in the environment: They have been found in diverse habitats ranging from ocean sediment to decaying cacti to insect bodies and on all continents including Antarctica. However, environmental killer yeasts are poorly studied compared with laboratory and domesticated killer yeasts. Killer yeasts secrete so-called killer toxins that inhibit nearby sensitive yeasts, and the toxins are frequently assumed to be tools for interference competition in diverse yeast communities. The diversity and ubiquity of killer yeasts imply that interference competition is crucial for shaping yeast communities. Additionally, these toxins may have ecological functions beyond use in interference competition. This review introduces readers to killer yeasts in environmental systems, with a focus on what is and is not known about their ecology and evolution. It also explores how results from experimental killer systems in laboratories can be extended to understand how competitive strategies shape yeast communities in nature. Overall, killer yeasts are likely to occur everywhere yeasts are found, and the killer phenotype has the potential to radically shape yeast diversity in nature.

Volatile organic compounds (VOCs) produced by biocontrol yeasts

The Volatile organic compounds (VOCs) produced by biocontrol yeast strains which belong to the Wickerhamomyces anomalus, Metschnikowia pulcherrima, Aureobasidium pullulans and Saccharomyces cerevisiae species were identified by solid phase microextraction (SPME) coupled with Gas Chromatography-Mass Spectrometry (GC-MS). Alcohols (ethyl alcohol, 3-methyl-1-butanol and phenylethyl alcohol) and esters (ethyl acetate and isoamyl acetate) were found to be the main VOCs emitted by the yeast strains, which had different production rate over a 16-day period. In addition, the tested yeast strains showed a remarkable ability to consume oxygen and to produce high percentages of carbon dioxide over a 5 days incubation period in a model system. The yeast strains, which were proven to very efficiently suppress in vivo the growth of postharvest fungal by VOCs, also quickly produced high percentages of ethyl acetate and carbon dioxide. . For all these reasons, we believe that the level of yeast biocontrol efficacy through the production of volatiles could be the result of a synergistic effect between VOCs and carbon dioxide in the packaging environment.

Single and combined effects of acetic acid, furfural, and sugars on the growth of the pentose-fermenting yeast Meyerozyma guilliermondii

The tolerance of the pentose-fermenting yeast Meyerozyma guilliermondii to the inhibitors released after the biomass hydrolysis, such as acetic acid and furfural, was surveyed. We first verified the effects of acetic acid and cell concentrations and initial pH on the growth of a M. guilliermondii strain in a semi-synthetic medium containing acetic acid as the sole carbon source. Second, the single and combined effects of furfural, acetic acid, and sugars (xylose, arabinose, and glucose) on the sugar uptake, cell growth, and ethanol production were also analysed. Growth inhibition occurred in concentrations higher than 10.5 g l-1 acetic acid and initial pH 3.5. The maximum specific growth rate (µ) was 0.023 h-1 and the saturation constant (ks) was 0.75 g l-1 acetic acid. Initial cell concentration also influenced µ. Acetic acid (initial concentration 5 g l-1) was co-consumed with sugars even in the presence of 20 mg l-1 furfural without inhibition to the yeast growth. The yeast grew and fermented sugars in a sugar-based medium with acetic acid and furfural in concentrations much higher than those usually found in hemicellulosic hydrolysates.

Biocontrol ability of killer yeasts (Saccharomyces cerevisiae) isolated from wine against Colletotrichum gloeosporioides on grape

A total of 216 killer yeasts Saccharomyces cerevisiae, isolated from wine, were evaluated in controlling Colletotrichum gloeosporioides, a pre-harvest anthracnose agent of grape. Three of these yeast isolates were tested positive for antagonizing C. gloeosporioides and were further evaluated for their mechanisms as biological control agents (BCAs): production of antifungal compounds, production of hydrolytic enzymes, inhibition of C. gloeosporioides conidia germination, colonization on grape berry, and efficiency in controlling anthracnose of grape. The results showed that all three S. cerevisiae isolates produced antifungal compounds, inhibited C. gloeosporioides conidia germination and produced β-1,3-glucanase and chitinase. All isolates colonized grape berry in large quantities and controlled C. gloeosporioides when artificially inoculated on grape berry. Among the three isolates, application of isolate GA8 resulted in 69.7% of disease reductions for C. gloeosporioides on grape berry. The antagonistic isolates of S. cerevisiae could represent important BCAs of anthracnose of grape caused by C. gloeosporioides that are responsible for economic losses in viticulture.

Mosquitoes can harbour yeasts of clinical significance and contribute to their environmental dissemination

There is still a lack of studies on fungal microbiota in mosquitoes, compared with the number available on bacterial microbiota. This study reports the identification of yeasts of clinical significance in laboratory mosquito species: Anopheles gambiae, Anopheles stephensi, Culex quinquefasciatus, Aedes albopictus and Aedes aegypti. Among the yeasts isolated, they focused on the opportunistic pathogen Candida parapsilosis, since there is a need to better understand breakthrough candidaemia with resistance to the usual antifungals, which requires careful consideration in the broad-spectrum therapy, as documented in many clinical reports. C. parapsilosis occurs widely and has been isolated from diverse sources, including insects, which may contribute to its dissemination. In this study, it was isolated from the gut of An. gambiae and its presence in developmental stages and organs of different mosquito species was studied. Our results indicated that there was a stable association between C. parapsilosis and reared mosquitoes during the entire life cycle, and in adult male and female gut and gonads. A wide occurrence of C. parapsilosis was also documented in several populations of wild mosquitoes. Based on these findings, it can be said that mosquitoes might participate in the spreading of this opportunistic pathogen, not only as a carrier.

Native Killer Yeasts as Biocontrol Agents of Postharvest Fungal Diseases in Lemons

Economic losses caused by postharvest diseases represent one of the main problems of the citrus industry worldwide. The major diseases affecting citrus are the "green mold" and "blue mold", caused by Penicillium digitatum and P. italicum, respectively. To control them, synthetic fungicides are the most commonly used method. However, often the emergence of resistant strains occurs and their use is becoming more restricted because of toxic effects and environmental pollution they generate, combined with trade barriers to international markets. The aim of this work was to isolate indigenous killer yeasts with antagonistic activity against fungal postharvest diseases in lemons, and to determine their control efficiency in in vitro and in vivo assays. Among 437 yeast isolates, 8.5% show to have a killer phenotype. According to molecular identification, based on the 26S rDNA D1/D2 domain sequences analysis, strains were identified belonging to the genera Saccharomyces, Wickerhamomyces, Kazachstania, Pichia, Candida and Clavispora. Killers were challenged with pathogenic molds and strains that caused the maximum in vitro inhibition of P. digitatum were selected for in vivo assays. Two strains of Pichia and one strain of Wickerhamomyces depicted a significant protection (p <0.05) from decay by P. digitatum in assays using wounded lemons. Thus, the native killer yeasts studied in this work showed to be an effective alternative for the biocontrol of postharvest fungal infections of lemons and could be promising agents for the development of commercial products for the biological control industry.

A strain of Meyerozyma guilliermondii isolated from sugarcane juice is able to grow and ferment pentoses in synthetic and bagasse hydrolysate media

The search for new microbial strains that are able to withstand inhibitors released from hemicellulosic hydrolysis and are also still able to convert sugars in ethanol/xylitol is highly desirable. A yeast strain isolated from sugarcane juice and identified as Meyerozyma guilliermondii was evaluated for the ability to grow and ferment pentoses in synthetic media and in sugarcane bagasse hydrolysate. The yeast grew in xylose, arabinose and glucose at the same rate at an initial medium pH of 5.5. At pH 4.5, the yeast grew more slowly in arabinose. There was no sugar exhaustion within 60 h. At higher xylose concentrations with a higher initial cell concentration, sugar was exhausted within 96 h at pH 4.5. An increase of 350 % in biomass was obtained in detoxified hydrolysates, whereas supplementation with 3 g/L yeast extract increased biomass production by approximately 40 %. Ethanol and xylitol were produced more significantly in supplemented hydrolysates regardless of detoxification. Xylose consumption was enhanced in supplemented hydrolysates and arabinose was consumed only when xylose and glucose were no longer available. Supplementation had a greater impact on ethanol yield and productivity than detoxification; however, the product yields obtained in the present study are still much lower when compared to other yeast species in bagasse hydrolysate. By the other hand, the fermentation of both xylose and arabinose and capability of withstanding inhibitors are important characteristics of the strain assayed.

Saccharomyces cerevisiae: A novel and efficient biological control agent for Colletotrichum acutatum during pre-harvest

In this study, we evaluated the efficiency of six isolates of Saccharomyces cerevisiae in controlling Colletotrichum acutatum, the causal agent of postbloom fruit drop that occur in pre-harvest citrus. We analyzed the mechanisms of action involved in biological control such as: production of antifungal compounds, nutrient competition, detection of killer activity, and production of hydrolytic enzymes of the isolates of S. cerevisiae on C. acutatum and their efficiency in controlling postbloom fruit drop on detached citrus flowers. Our results showed that all six S. cerevisiae isolates produced antifungal compounds, competed for nutrients, inhibited pathogen germination, and produced killer activity and hydrolytic enzymes when in contact with the fungus wall. The isolates were able to control the disease when detached flowers were artificially inoculated, both preventively and curatively. In this work we identified a novel potential biological control agent for C. acutatum during pre-harvest. This is the first report of yeast efficiency for the biocontrol of postbloom fruit drop, which represents an important contribution to the field of biocontrol of diseases affecting citrus populations worldwide.