Production of a novel and cold-active killer toxin by Mrakia frigida 2E00797 isolated from sea sediment in Antarctica

Biobank of fungi from marine and terrestrial Antarctic environments

Harsh and extreme environments, such as Antarctica, offer unique opportunities to explore new microbial taxa and biomolecules. Given the limited knowledge on microbial diversity, this study aimed to compile, analyze and compare a subset of the biobank of Antarctic fungi maintained at the UNESP's Central of Microbial Resources (CRM-UNESP). A total of 711 isolates (240 yeasts and 471 filamentous fungi) from marine and terrestrial samples collected at King George Island (South Shetland Islands, Antarctica) were used with the primary objective of investigating their presence in both marine and terrestrial environments. Among the yeasts, 13 genera were found, predominantly belonging to the phylum Basidiomycota. Among the filamentous fungi, 34 genera were represented, predominantly from the phylum Ascomycota. The most abundant genera in the marine samples were Metschnikowia, Mrakia, and Pseudogymnoascus, while in the terrestrial samples, they were Pseudogymnoascus, Leucosporidium, and Mortierella. Most of the genera and species of the CRM-UNESP biobank of Antarctic fungi are being reported as an important target for biotechnological applications. This study showed the relevance of the CRM-UNESP biobank, highlighting the importance of applying standard methods for the preservation of the biological material and associated data (BMaD), as recommended in national and international standards.

A global meta-analysis of animal manure application and soil microbial ecology based on random control treatments

The processes involved in soil domestication have altered the soil microbial ecology. We examined the question of whether animal manure application affects the soil microbial ecology of farmlands. The effects of global animal manure application on soil microorganisms were subjected to a meta-analysis based on randomized controlled treatments. A total of 2303 studies conducted in the last 30 years were incorporated into the analysis, and an additional 45 soil samples were collected and sequenced to obtain 16S rRNA and 18S rRNA data. The results revealed that manure application increased soil microbial biomass. Manure application alone increased bacterial diversity (M-Z: 7.546 and M-I: 8.68) and inhibited and reduced fungal diversity (M-Z: -1.15 and M-I: -1.03). Inorganic fertilizer replaced cattle and swine manure and provided nutrients to soil microorganisms. The soil samples of the experimental base were analyzed, and the relative abundances of bacteria and fungi were altered compared with no manure application. Manure increased bacterial diversity and reduced fungal diversity. Mrakia frigida and Betaproteobacteriales, which inhibit other microorganisms, increased significantly in the domesticated soil. Moreover, farm sewage treatments resulted in a bottleneck in the manure recovery rate that should be the focus of future research. Our results suggest that the potential risks of restructuring the microbial ecology of cultivated land must be considered.

Kinetics and mathematical model of killer/sensitive interaction under different physicochemical conditions of must/wine: a study from a biological point of view

Fermentation of grape must to wine is carried out by a complex microbial mixture, which also involves spoilage yeasts of wine. The latter yeasts produce organoleptic changes that cause significant economic losses to the wine industry. SO₂ is traditionally used to control this spoilage populations, but because of its harmful effects on human health, biocontrol has emerged as an alternative treatment. Although studies have been carried out to select biocontroller yeasts and examine their underlying mechanisms of action, reports on their application have not been published yet. To better understand the interaction and the successful application of biocontrol, the use of mathematical models, among other methods, is important, as they facilitate the prediction of success or failure of the antagonist. The objective of the present study was to use an existing mathematical model to obtain information about the yeast's interaction assayed and to validate its predictive use under different physicochemical conditions during the wine fermentation, and eventually predict biocontrol kinetics. The mathematical model was applied to the fermentation conditions and provided information on the kinetic parameters of the biocontrol interaction and allowed interpretations about other parameters. The model was applied in the different physicochemical conditions for the biocontrol and did not fit correctly to experimental data, and therefore an improvement was proposed which was successful and presented new hypotheses.

Biocontrol yeasts: mechanisms and applications

Yeasts occur in all environments and have been described as potent antagonists of various plant pathogens. Due to their antagonistic ability, undemanding cultivation requirements, and limited biosafety concerns, many of these unicellular fungi have been considered for biocontrol applications. Here, we review the fundamental research on the mechanisms (e.g., competition, enzyme secretion, toxin production, volatiles, mycoparasitism, induction of resistance) by which biocontrol yeasts exert their activity as plant protection agents. In a second part, we focus on five yeast species (Candida oleophila, Aureobasidium pullulans, Metschnikowia fructicola, Cryptococcus albidus, Saccharomyces cerevisiae) that are or have been registered for the application as biocontrol products. These examples demonstrate the potential of yeasts for commercial biocontrol usage, but this review also highlights the scarcity of fundamental studies on yeast biocontrol mechanisms and of registered yeast-based biocontrol products. Yeast biocontrol mechanisms thus represent a largely unexplored field of research and plentiful opportunities for the development of commercial, yeast-based applications for plant protection exist.

Purification and molecular characterization of a Metschnikowia saccharicola killer toxin lethal to a crab pathogenic yeast

The marine yeast strain Metschnikowia saccharicola DD21-2, isolated from sediments in the Yalu River, produces a killer toxin with a lethal effect on Metschnikowia bicuspidate strain WCY, a pathogenic yeast strain that infects crabs. In this study, the killer toxin was purified and characterized. After sequential purification, the purity of the killer toxin was increased 72.2-fold over the purity of the yeast cell culture supernatant. The molecular weight of the purified killer toxin was 47.0 kDa. The optimal pH and temperature for killing activity were 5.5°C and 16°C, respectively. The killing activity was stable over a pH range of 4.0-6.5 and temperature range of 0°C-40°C. The purified killer toxin was only effective against toxin-sensitive integral cells and had no killing effect on the protoplasts of toxin-sensitive cells. When exerting the killing effect, the toxin bind to a cell wall receptor of the treated strain, disrupted cell wall integrity and eventually caused death. The amino acid sequence identified by mass spectroscopy indicated that the purified killer toxin might be a protein kinase, but did not show β-1,3-glucanase activity, consistent with the laminarin hydrolysis results. These findings provide a basis for disease prevention and control in marine aquaculture.

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.

Metagenomic analysis of orange colored protrusions from the muscle of Queen Conch Lobatus gigas (Linnaeus, 1758)

The endangered marine gastropod, Lobatus gigas, is an important fishery resource in the Caribbean region. Microbiological and parasitological research of this species have been poorly addressed despite its role in ecological fitness, conservation status and prevention of potential pathogenic infections. This study identified taxonomic groups associated with orange colored protrusions in the muscle of queen conchs using histological analysis, 454 pyrosequencing, and a combination of PCR amplification and automated Sanger sequencing. The molecular approaches indicate that the etiological agent of the muscle protrusions is a parasite belonging to the subclass Digenea. Additionally, the scope of the molecular technique allowed the detection of bacterial and fungi clades in the assignment analysis. This is the first evidence of a digenean infection in the muscle of this valuable Caribbean resource.

Production, Characterization, and Antimicrobial Activity of Mycocin Produced by Debaryomyces hansenii DSMZ70238

The present study was conducted to estimate the antimicrobial activity and the potential biological control of the killer toxin produced by D. hansenii DSMZ70238 against several pathogenic microorganisms. In this study, the effects of NaCl, pH, and temperature, killer toxin production, and antimicrobial activity were studied. The results showed that the optimum inhibitory effect of killer toxin was at 8% NaCl, and the diameters of clear zones were 20, 22, 22, 21, 14, and 13 mm for Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, Streptococcus pyogenes, Candida albicans, and Candida neoformans, respectively. The largest inhibition zones were observed at pH 4.5 with inhibition zone of 16, 18, 17, 18, 11, and 12 mm for the same microorganisms. The results also showed that 25°C is the optimal temperature for toxin killing activity against all targeted microorganisms. In addition, the activity of killer toxin significantly inhibited the growth of fungal mycelia for all target pathogenic fungi and the percentages of inhibition were 47.77, 48.88, 52.22, and 61.11% for Trichophyton rubrum, Alternaria alternata, Trichophyton concentricum, and Curvularia lunata, respectively. The results showed the highest growth rate of D. hansenii DSMZ70238 under condition of 8% NaCl concentration, pH 4.5, and 25°C for 72 h.

The Biology of Pichia membranifaciens Killer Toxins

The killer phenomenon is defined as the ability of some yeast to secrete toxins that are lethal to other sensitive yeasts and filamentous fungi. Since the discovery of strains of Saccharomyces cerevisiae capable of secreting killer toxins, much information has been gained regarding killer toxins and this fact has substantially contributed knowledge on fundamental aspects of cell biology and yeast genetics. The killer phenomenon has been studied in Pichia membranifaciens for several years, during which two toxins have been described. PMKT and PMKT2 are proteins of low molecular mass that bind to primary receptors located in the cell wall structure of sensitive yeast cells, linear (1→6)-β-d-glucans and mannoproteins for PMKT and PMKT2, respectively. Cwp2p also acts as a secondary receptor for PMKT. Killing of sensitive cells by PMKT is characterized by ionic movements across plasma membrane and an acidification of the intracellular pH triggering an activation of the High Osmolarity Glycerol (HOG) pathway. On the contrary, our investigations showed a mechanism of killing in which cells are arrested at an early S-phase by high concentrations of PMKT2. However, we concluded that induced mortality at low PMKT2 doses and also PMKT is indeed of an apoptotic nature. Killer yeasts and their toxins have found potential applications in several fields: in food and beverage production, as biocontrol agents, in yeast bio-typing, and as novel antimycotic agents. Accordingly, several applications have been found for P. membranifaciens killer toxins, ranging from pre- and post-harvest biocontrol of plant pathogens to applications during wine fermentation and ageing (inhibition of Botrytis cinerea, Brettanomyces bruxellensis, etc.).

Identification and characterization of yeasts isolated from sedimentary rocks of Union Glacier at the Antarctica

The study of the yeasts that inhabit cold environments, such as Antarctica, is an active field of investigation oriented toward understanding their ecological roles in these ecosystems. In a great part, the interest in cold-adapted yeasts is due to several industrial and biotechnological applications that have been described for them. The aim of this work was to isolate and identify yeasts from sedimentary rock samples collected at the Union Glacier, Antarctica. Furthermore, the yeasts were physiologically characterized, including the production of metabolites of biotechnological interest. The yeasts isolated that were identified at the molecular level belonged to genera Collophora (1 isolate), Cryptococcus (2 isolates), Sporidiobolus (4 isolates), Sporobolomyces (1 isolate) and Torrubiella (2 isolates). The majority of yeasts were basidiomycetous and psychrotolerant. By cross-test assays for anti-yeast activity, it was determined that Collophora sp., Sporidiobolus salmonicolor, and Sporobolomyces roseus secreted a protein factor that kills Sporidiobolus metaroseus. The colored yeasts Sp. salmonicolor, Sp. metaroseus and Collophora sp. produced several carotenoid pigments that were identified as 2,3 dihydroxy-γ-carotene, -carotene, 4-ketotorulene, torulene β-cryptoxanthin and spirilloxanthin. Concerning analysis of mycosporines, these metabolites were only found in the yeasts Torrubiella sp. and Cryptococcus sp. T11-10-1. Furthermore, the yeasts were evaluated for the production of extracellular hydrolytic activities. Of the twelve activities analyzed, alkaline phosphatase, invertase, gelatinase, cellulase, amylase, and protease enzyme activities were detected. The yeasts Cryptococcus sp. T11-10-1 and Sporidiobolus metaroseus showed the highest number of different enzyme activities.

A Novel Saccharomyces cerevisiae Killer Strain Secreting the X Factor Related to Killer Activity and Inhibition of S. cerevisiae K1, K2 and K28 Killer Toxins

It was determined that Kx strains secrete an X factor which can inhibit all known Saccharomyces cerevisiae killer toxins (K1, K2, K28) and some toxins of other yeast species-the phenomenon not yet described in the scientific literature. It was shown that Kx type yeast strains posess a killer phenotype producing small but clear lysis zones not only on the sensitive strain α'1 but also on the lawn of S. cerevisiae K1, K2 and K28 type killer strains at temperatures between 20 and 30 °C. The pH at which killer/antikiller effect of Kx strain reaches its maximum is about 5.0-5.2. The Kx yeast were identified as to belong to S. cerevisiae species. Another newly identified S. cerevisiae killer strain N1 has killer activity but shows no antikilller properties against standard K1, K2 and K28 killer toxins. The genetic basis for Kx killer/antikiller phenotype was associated with the presence of M-dsRNA which is bigger than M-dsRNA of standard S. cerevisiae K1, K2, K28 type killer strains. Killer and antikiller features should be encoded by dsRNA. The phenomenon of antikiller (inhibition) properties was observed against some killer toxins of other yeast species. The molecular weight of newly identified killer toxins which produces Kx type strains might be about 45 kDa.

Rhodotorula portillonensis sp. nov., a basidiomycetous yeast isolated from Antarctic shallow-water marine sediment

During the characterization of the mycobiota associated with shallow-water marine environments from Antarctic sea, a novel pink yeast species was isolated. Sequence analysis of the D1/D2 domain of the LSU rDNA gene and 5.8S-ITS regions revealed that the isolated yeast was closely related to Rhodotorula pallida CBS 320(T) and Rhodotorula benthica CBS 9124(T). On the basis of morphological, biochemical and physiological characterization and phylogenetic analyses, a novel basidiomycetous yeast species, Rhodotorula portillonensis sp. nov., is proposed. The type strain is Pi2(T) ( = CBS 12733(T)  = CECT 13081(T)) which was isolated from shallow-water marine sediment in Fildes Bay, King George Island, Antarctica.

A novel killer toxin produced by the marine-derived yeast Wickerhamomyces anomalus YF07b

In our previous study, it was found that the killer toxin produced by the marine-derived yeast Wickerhamomyces anomalus YF07b has both killing activity and β-1,3-glucanase activity and the molecular mass of it is 47.0 kDa. In this study, the same yeast strain was found to produce another killer toxin which only had killing activity against some yeast strains, but had no β-1,3-glucanase activity and the molecular mass of the purified killer toxin was 67.0 kDa. The optimal pH, temperature and NaCl concentration for action of the purified killer toxin were 3.5, 16 °C and 4.0 % (w/v), respectively. The purified killer toxin could be bound by the whole sensitive yeast cells, but was not bound by manann, chitin and β-1,3-glucan. The purified killer toxin had killing activity against Yarrowia lipolytica, Saccharomyces cerevisiae, Metschnikowia bicuspidata WCY, Candida tropicalis, Candida albicans and Kluyveromyces aestuartii. Lethality of the sensitive cells treated by the newly purified killer toxin from W. anomalus YF07b involved disruption of cellular integrity by permeabilizing cytoplasmic membrane function.

Disruption of the gene encoding β-1, 3-glucanase in marine-derived Williopsis saturnus WC91-2 enhances its killer toxin activity

As the β-1, 3-glucanase produced by the marine-derived Williopsis saturnus WC91-2 could inhibit the activity of the killer toxin produced by the same yeast, the WsEXG1 gene encoding exo-β-1, 3-glucanase in W. saturnus WC91-2 was disrupted. The disruptant WC91-2-2 only produced a trace amount of β-1, 3-glucanase but had much higher activity of killer toxin than W. saturnus WC91-2. After the disruption of the WsEXG1 gene, the expression of the gene was significantly decreased from 100% in the cells of W. saturnus WC91-2 to 27% in the cells of the disruptant WC91-2-2 while the expression of the killer toxin gene in W. saturnus WC91-2 and the disruptant WC91-2-2 was almost the same. During 2-l fermentation, the disruptant WC91-2-2 could produce the highest amount of killer toxin (the size of the inhibition zone was 22 ± 0.7 mm) within 36 h when the cell growth reached the middle of the log phase.

Psychrophilic yeasts from worldwide glacial habitats: diversity, adaptation strategies and biotechnological potential

Glacial habitats (cryosphere) include some of the largest unexplored and extreme biospheres on Earth. These habitats harbor a wide diversity of psychrophilic prokaryotic and eukaryotic microorganisms. These highly specialized microorganisms have developed adaptation strategies to overcome the direct and indirect life-endangering influence of low temperatures. For many years Antarctica has been the geographic area preferred by microbiologists for studying the diversity of psychrophilic microorganisms (including yeasts). However, there have been an increasing number of studies on psychrophilic yeasts sharing the non-Antarctic cryosphere. The present paper provides an overview of the distribution and adaptation strategies of psychrophilic yeasts worldwide. Attention is also focused on their biotechnological potential, especially on their exploitation as a source of cold-active enzymes and for bioremediation purposes.

Purification, characterization and gene cloning of the killer toxin produced by the marine-derived yeast Williopsis saturnus WC91-2

As the killer toxin produced by Williopsis saturnus WC91-2 could kill many sensitive yeast strains, including the pathogenic ones, the extracellular killer toxin in the supernatant of cell culture of the marine yeast strain was purified and characterized. The molecular mass of the purified killer toxin was estimated to be 11.0 kDa according to the data from SDS-PAGE. The purified killer toxin had killing activity, but could not hydrolyze laminarin. The optimal conditions for action of the purified killer toxin against the pathogenic yeast Metschnikowia bicuspidate WCY were the assay medium with 10% NaCl, pH 3-3.5 and temperature 16 °C. The gene encoding the killer toxin from the marine killer yeast WC91-2 was cloned and the ORF of the gene was 378 bp. The deduced protein from the cloned gene encoding the killer toxin had 125 amino acids with calculated molecular weight of 11.6 kDa. It was also found that the N-terminal amino acid sequence of the purified killer toxin had the same corresponding sequence deduced from the cloned killer toxin gene in this marine yeast, confirming that the purified killer toxin was indeed encoded by the cloned gene.