Effect of salt on the killer phenotype of yeasts from olive brines

Screening a 681-membered yeast collection for the secretion of proteins with antifungal activity

Fungal pathogens pose a threat to human health and food security. Few antifungals are available and resistance to all has been reported. Novel strategies to control plant and human pathogens as well as food spoilers are urgently required. Environmental yeasts provide a functionally diverse, yet underexploited potential for fungal control based on their natural competition via the secretion of proteins and other small molecules such as iron chelators, volatile organic compounds or biosurfactants. However, there is a lack of standardized workflows to systematically access application-relevant yeast-based compounds and understand their molecular functioning. Towards this goal, we developed a workflow to identify and characterize yeast isolates that are active against spoilage yeasts and relevant human and plant pathogens, herein focusing on discovering yeasts that secrete antifungal proteins. The workflow includes the classification of the secreted molecules and cross-comparison of their antifungal capacity using an independent synthetic calibrant. Our workflow delivered a collection of 681 yeasts of which 212 isolates (31 %) displayed antagonism against at least one target strain. While 57.5 % of the active yeasts showed iron-depended antagonism, likely due to pulcherrimin-like iron chelators, 31.7 % secreted antifungal proteins. Those yeast candidates clustered within twelve OTUs, showed narrow and broad target spectra, and several showed a broad pH and temperature activity profile. Given the tools for yeast biotechnology and protein engineering available, our collection can serve as a rich starting point for genetic and molecular characterization of the various antifungal phenotypes, their mode of action and their future exploitation.

Impact of lactic acid bacteria inoculation on fungal diversity during Spanish-style green table olive fermentations

This study delved into the evolution of fungal population during the fermentation of Spanish-style green table olives (Manzanilla cultivar), determining the influence of different factors such as fermentation matrix (brine or fruit) or the use of a lactic acid bacteria inoculum, on its distribution. The samples (n = 24) were directly obtained from industrial fermentation vessels with approximately 10.000 kg of fruits and 6.000 L of brines. Our findings showcased a synchronized uptick in lactic acid bacteria counts alongside fungi proliferation. Metataxonomic analysis of the Internal Transcribed Spacer (ITS) region unearthed noteworthy disparities across different fermentation time points (0, 24, and 83 days). Statistical analysis pinpointed two Amplicon Sequence Variants (ASV), Candida and Aureobasidium, as accountable for the observed variances among the different fermentation time samples. Notably, Candida exhibited a marked increase during 83 days of fermentation, opposite to Aureobasidium, which demonstrated a decline. Fungal biodiversity was slightly higher in brines than in fruits, whilst no effect of inoculation was noticed. At the onset of fermentation, prominently detected genera were also Mycosphaerella (19.82 %) and Apohysomyces (16.31 %), hitherto unreported in the context of table olive processing. However, their prevalence dwindled to nearly negligible levels from 24th day fermentation onwards (<2 %). On the contrary, they were replaced by the fermentative yeasts Saccharomyces and Isstachenkia. Results obtained in this work will be useful for designing new strategies for better control of table olive fermentations.

Utilization of salt-rich by-products from the dairy industry as feedstock for recombinant protein production by Debaryomyces hansenii

The dairy industry processes vast amounts of milk and generates high amounts of secondary by-products, which are still rich in nutrients (high Chemical Oxygen Demand (COD) and Biochemical Oxygen Demand (BOD) levels) but contain high concentrations of salt. The current European legislation only allows disposing of these effluents directly into the waterways with previous treatment, which is laborious and expensive. Therefore, as much as possible, these by-products are reutilized as animal feed material and, if not applicable, used as fertilizers adding phosphorus, potassium, nitrogen, and other nutrients to the soil. Finding biological alternatives to revalue dairy by-products is of crucial interest in order to improve the utilization of dry dairy matter and reduce the environmental impact of every litre of milk produced. Debaryomyces hansenii is a halotolerant non-conventional yeast with high potential for this purpose. It presents some beneficial traits - capacity to metabolize a variety of sugars, tolerance to high osmotic environments, resistance to extreme temperatures and pHs - that make this yeast a well-suited option to grow using complex feedstock, such as industrial waste, instead of the traditional commercial media. In this work, we study for the first time D. hansenii's ability to grow and produce a recombinant protein (YFP) from dairy saline whey by-products. Cultivations at different scales (1.5, 100 and 500 ml) were performed without neither sterilizing the medium nor using pure water. Our results conclude that D. hansenii is able to perform well and produce YFP in the aforementioned salty substrate. Interestingly, it is able to outcompete other microorganisms present in the waste without altering its cell performance or protein production capacity.

The killer yeast Wickerhamomyces anomalus Cf20 exerts a broad anti-Candida activity through the production of killer toxins and volatile compounds

Candidiasis is a group of opportunistic infections caused by yeast of the genus Candida. The appearance of drug resistance and the adverse effects of current antifungal therapies require the search for new, more efficient therapeutic alternatives. Killer yeasts have aroused as suitable candidates for mining new antifungal compounds. Killer strains secrete antimicrobial proteins named killer toxins, with promissory antifungal activity. Here we found that the killer yeast Wickerhamomyces anomalus Cf20 and its cell-free supernatant (CFS) inhibited six pathogenic strains and one collection strain of Candida spp. The inhibition is mainly mediated by secreted killer toxins and, to a lesser extent, by volatile compounds such as acetic acid and ethyl acetate. A new large killer toxin (>180 kDa) was purified, which exerted 70-74% of the total CFS anti-Candida activity, and the previously described glucanase KTCf20 was inhibitory in a lesser extent as well. In addition, we demonstrated that Cf20 possesses the genes encoding for the β-1,3-glucanases WaExg1 and WaExg2, proteins with extensively studied antifungal activity, particularly WaExg2. Finally, the 10-fold concentrated CFS exerted a high candidacidal effect at 37°C, completely inhibiting the fungal growth, although the nonconcentrated CFS (RCF 1) had very limited fungistatic activity at this temperature. In conclusion, W. anomalus Cf20 produces different low and high molecular weight compounds with anti-Candida activity that could be used to design new therapies for candidiasis and as a source for novel antimicrobial compounds as well.

Microbiota profiling and screening of the lipase active halotolerant yeasts of the olive brine

Searching for novel enzymes that could be active in organic solvents has become an area of interest in recent years. Olive brine naturally provides a suitable environment for the survival of halophilic and acidophilic microorganisms and the resulting genome is thought to be a gene source for determining the halophilic and acidophilic proteins that are active in a non-aqueous organic solvent medium, and so it has been used in several biotechnological and industrial applications. In this study, microbial analysis of natural, cracked green olive brine from the southern region of Turkey has been made by next-generation sequencing of the brine metagenome for the first time in the literature. The number of reads assigned to fungal operational taxonomic units was the highest percentage (73.04%) with the dominant representation of Ascomycota phylum (99% of fungi). Bacterial OTU was 3.56% of the reads and Proteobacteria phylum was 65% of the reads. The lipase production capacity of the yeasts that were grown on the media containing elevated concentrations of NaCl (1-3 M) was determined on a Rhodamine B-including medium. Molecular identification of the selected yeasts was performed and 90% of sequenced yeasts had a high level of similarity with Candida diddensiae, whereas 10% showed similarity to Candida boidinii. The hydrolytic lipase activities using olive oil were analyzed and both yeasts showed cell-bound lipase activity at pH 3.0.

Antimicrobial mechanisms and applications of yeasts

Yeasts and humans have had a close relationship for millenia. Yeast have been used for food production since the first human societies. Since then, alternative uses have been discovered. Nowadays, antibiotic resistance constitutes a pressing need worldwide. In order to overcome this threat, one of the most important strategies is the search for new antimicrobials in natural sources. Moreover, biopreservation based on natural sources has emerged as an alternative to more common chemical preservatives. Yeasts constitute an underexploited source of antagonistic activity against other microorganisms. Here, we compile a summary of the antagonistic activity of yeast origin against other yeast and other microorganisms, such as bacteria or parasites. We present the mechanisms of action used by yeasts to display these activities. We also provide applications of these antagonistic activities in food industry and agriculture, medicine and veterinary, where yeast promise to play a pivotal role in the near future.

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.

Yeast killer toxins: from ecological significance to application

Killer toxins are proteins that are often glycosylated and bind to specific receptors on the surface of their target microorganism, which is then killed through a target-specific mode of action. The killer phenotype is widespread among yeast and about 100 yeast killer species have been described to date. The spectrum of action of the killer toxins they produce targets spoilage and pathogenic microorganisms. Thus, they have potential as natural antimicrobials in food and for biological control of plant pathogens, as well as therapeutic agents against animal and human infections. In spite of this wide range of possible applications, their exploitation on the industrial level is still in its infancy. Here, we initially briefly report on the biodiversity of killer toxins and the ecological significance of their production. Their actual and possible applications in the agro-food industry are discussed, together with recent advances in their heterologous production and the manipulation for development of peptide-based therapeutic agents.

Edible Films and Coatings as Carriers of Living Microorganisms: A New Strategy Towards Biopreservation and Healthier Foods

Edible films and coatings have been extensively studied in recent years due to their unique properties and advantages over more traditional conservation techniques. Edible films and coatings improve shelf life and food quality, by providing a protective barrier against physical and mechanical damage, and by creating a controlled atmosphere and acting as a semipermeable barrier for gases, vapor, and water. Edible films and coatings are produced using naturally derived materials, such as polysaccharides, proteins, and lipids, or a mixture of these materials. These films and coatings also offer the possibility of incorporating different functional ingredients such as nutraceuticals, antioxidants, antimicrobials, flavoring, and coloring agents. Films and coatings are also able to incorporate living microorganisms. In the last decade, several works reported the incorporation of bacteria to confer probiotic or antimicrobial properties to these films and coatings. The incorporation of probiotic bacteria in films and coatings allows them to reach the consumers' gut in adequate amounts to confer health benefits to the host, thus creating an added value to the food product. Also, other microorganisms, either bacteria or yeast, can be incorporated into edible films in a biocontrol approach to extend the shelf life of food products. The incorporation of yeasts in films and coatings has been suggested primarily for the control of the postharvest disease. This work provides a comprehensive review of the use of edible films and coatings for the incorporation of living microorganisms, aiming at the biopreservation and probiotic ability of food products.

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.).

Bioprotective Role of Yeasts

The yeasts constitute a large group of microorganisms characterized by the ability to grow and survive in different and stressful conditions and then to colonize a wide range of environmental and human ecosystems. The competitive traits against other microorganisms have attracted increasing attention from scientists, who proposed their successful application as bioprotective agents in the agricultural, food and medical sectors. These antagonistic activities rely on the competition for nutrients, production and tolerance of high concentrations of ethanol, as well as the synthesis of a large class of antimicrobial compounds, known as killer toxins, which showed clearly a large spectrum of activity against food spoilage microorganisms, but also against plant, animal and human pathogens. This review describes the antimicrobial mechanisms involved in the antagonistic activity, their applications in the processed and unprocessed food sectors, as well as the future perspectives in the development of new bio-drugs, which may overcome the limitations connected to conventional antimicrobial and drug resistance.

Membrane bioreactor wastewater treatment plants reveal diverse yeast and protist communities of potential significance in biofouling

The yeast community was studied in a municipal full-scale membrane bioreactor wastewater treatment plant (MBR-WWTP). The unexpectedly high diversity of yeasts indicated that the activated sludge formed a suitable environment for them to proliferate, with cellular concentrations of 2.2 ± 0.8 × 10(3) CFU ml(-1). Sixteen species of seven genera were present in the biological reactor, with Ascomycetes being the most prevalent group (93%). Most isolates were able to grow in a synthetic wastewater medium, adhere to polyethylene surfaces, and develop biofilms of variable complexity. The relationship between yeast populations and the protists in the MBR-WWTP was also studied, revealing that some protist species preyed on and ingested yeasts. These results suggest that yeast populations may play a role in the food web of a WWTP and, to some extent, contribute to membrane biofouling in MBR systems.

Dynamic analysis of physiological properties of Torulaspora delbrueckii in wine fermentations and its incidence on wine quality

This work examines the physiology of a new commercial strain of Torulaspora delbrueckii in the production of red wine following different combined fermentation strategies. For a detailed comparison, several yeast metabolites and the strains implantation were measured over the entire fermentation period. In all fermentations in which T. delbrueckii was involved, the ethanol concentration was reduced; some malic acid was consumed; more pyruvic acid was released, and fewer amounts of higher alcohols were produced. The sensorial properties of final wines varied widely, emphasising the structure of wine in sequential fermentations with T. delbrueckii. These wines presented the maximum overall impression and were preferred by tasters. Semi-industrial assays were carried out confirming these differences at a higher scale. No important differences were observed in volatile aroma composition between fermentations. However, differences in mouthfeel properties were observed in semi-industrial fermentations, which were correlated with an increase in the mannoprotein content of red wines fermented sequentially with T. delbrueckii.

Antimicrobial and probiotic properties of yeasts: from fundamental to novel applications

The yeasts constitute a large and heterogeneous group of microorganisms that are currently attracting increased attention from scientists and industry. Numerous and diverse biological activities make them promising candidates for a wide range of applications not limited to the food sector. In addition to their major contribution to flavor development in fermented foods, their antagonistic activities toward undesirable bacteria, and fungi are now widely known. These activities are associated with their competitiveness for nutrients, acidification of their growth medium, their tolerance of high concentrations of ethanol, and release of antimicrobial compounds such as antifungal killer toxins or "mycocins" and antibacterial compounds. While the design of foods containing probiotics (microorganisms that confer health benefits) has focused primarily on Lactobacillus and Bifidobacterium, the yeast Saccharomyces cerevisiae var. boulardii has long been known effective for treating gastroenteritis. In this review, the antimicrobial activities of yeasts are examined. Mechanisms underlying this antagonistic activity as well as recent applications of these biologically active yeasts in both the medical and veterinary sectors are described.

Exoglucanase-encoding genes from three Wickerhamomyces anomalus killer strains isolated from olive brine

Wickerhamomyces anomalus killer strains are important for fighting pathogenic yeasts and for controlling harmful yeasts and bacteria in the food industry. Targeted disruption of key genes in β-glucan synthesis of a sensitive Saccharomyces cerevisiae strain conferred resistance to the toxins of W. anomalus strains BS91, BCA15 and BCU24 isolated from olive brine. Competitive inhibition of the killing activities by laminarin and pustulan refer to β-1,3- and β-1,6-glucans as the main primary toxin targets. The extracellular exoglucanase-encoding genes WaEXG1 and WaEXG2 from the three strains were sequenced and were found to display noticeable similarities to those from known potent W. anomalus killer strains.

Cell cycle arrest and apoptosis, two alternative mechanisms for PMKT2 killer activity

Pichia membranifaciens CYC 1086 secretes a unique 30kDa killer toxin (PMKT2) that inhibits a variety of spoilage yeasts and fungi of agronomical interest. The cytocidal effect of PMKT2 on Saccharomyces cerevisiae cells was studied. Metabolic events associated with the loss of S. cerevisiae viability caused by PMKT2 were qualitatively identical to those reported for K28 killer toxin activity, but different to those reported for PMKT. At higher doses, none of the cellular events accounting for the action of PMKT, the killer toxin secreted by P. membranifaciens CYC 1106, was observed for PMKT2. Potassium leakage, sodium influx and the decrease of intracellular pH were not among the primary effects of PMKT2. We report here that this protein is unable to form ion-permeable channels in liposome membranes, suggesting that channel formation is not the mechanism of cytotoxic action of PMKT2. Nevertheless, flow cytometry studies have revealed a cell cycle arrest at an early S-phase with an immature bud and pre-replicated 1n DNA content. By testing the sensitivity of cells arrested at different stages in the cell cycle, we hoped to identify the execution point for lethality more precisely. Cells arrested at the G1-phase by α-factor or arrested at G2-phase by the spindle poison methyl benzimidazol-2-yl-carbamate (MBC) were protected against the toxin. Cells released from the arrest in both cases were killed by PMKT2 at a similar rate. Nevertheless, cells released from MBC-arrest were able to grow for a short time, and then viability dropped rapidly. These findings suggest that cells released from G2-phase are initially able to divide, but die in the presence of PMKT2 after initiating the S-phase in a new cycle, adopting a terminal phenotype within that cycle. By contrast, low doses of PMKT and PMKT2 were able to generate the same cellular response. The evidence presented here shows that treating yeast with low doses of PMKT2 leads to the typical membranous, cytoplasmic, mitochondrial and nuclear markers of apoptosis, namely, the production of reactive oxygen species, DNA strand breaks, metacaspase activation and cytochrome c release.

Yeasts in table olive processing: desirable or spoilage microorganisms?

Yeasts are unicellular eukaryotic microorganisms isolated from many foods, and are commonly found in table olive processing where they can play a double role. On one hand, these microorganisms can produce spoilage of fruits due to the production of bad odours and flavours, the accumulation of CO(2) leading to swollen containers, the clouding of brines, the softening of fruits and the degradation of lactic acid, which is especially harmful during table olive storage and packaging. But on the other hand, fortunately, yeasts also possess desirable biochemical activities (lipase, esterase, β-glucosidase, catalase, production of killer factors, etc.) with important technological applications in this fermented vegetable. Recently, the probiotic potential of olive yeasts has begun to be evaluated because many species are able to resist the passage through the gastrointestinal tract and show beneficial effects on the host. In this way, yeasts may improve consumers' health by decreasing cholesterol levels, inhibiting pathogens, degrading non assimilated compounds, producing antioxidants and vitamins, adhering to intestinal cells or by maintaining epithelial barrier integrity. Many yeast species, usually also found in table olive processing, such as Wicherhamomyces anomalus, Saccharomyces cerevisiae, Pichia membranifaciens and Kluyveromyces lactis, have been reported to exhibit some of these properties. Thus, the selection of the most appropriate strains to be used as starters, alone or in combination with lactic acid bacteria, is a promising research line to develop in a near future which might improve the added value of the commercialized product.

Monitoring of killer yeast populations in mixed cultures: influence of incubation temperature of microvinifications samples

Killer yeasts are frequently used to combat and prevent contamination by wild-type yeasts during wine production and they can even dominate the wine fermentation. Stuck and sluggish fermentations can be caused by an unbalanced ratio of killer to sensitive yeasts in the bioreactor, and therefore it is important to determine the proportion of both populations. The aim of this study was to provide a simple tool to monitor killer yeast populations during controlled mixed microvinifications of killer and sensitive Saccharomyces cerevisiae. Samples were periodically extracted during vinification, seeded on Petri dishes and incubated at 25 and 37 °C; the latter temperature was assayed for possible inactivation of killer toxin production. Colonies developed under the described conditions were randomly transferred to killer phenotype detection medium. Significant differences in the killer/sensitive ratio were observed between both incubation temperatures in all microvinifications. These results suggest that 37 °C seems a better option to determine the biomass of sensitive yeasts, in order to avoid underestimation of sensitive cells in the presence of killer yeasts during fermentations. Incubation at a toxin-inhibiting temperature clearly showed the real ratio of killer to sensitive cells in fermentation systems.

Biocontrol of Penicillium nordicum growth and ochratoxin A production by native yeasts of dry cured ham

Twelve yeast strains isolated from the surface of Italian typical dry-cured hams, belonging to D. hansenii, D. maramus, C. famata, C. zeylanoides and H. burtonii species, and previously selected for their ability to grow in dry-cured ham-like substrates, were screened for antagonistic activity against a toxigenic strain of P. nordicum and inhibition of ochratoxin A (OTA) biosynthesis. On average, yeast inhibitory activity was lowered by increasing fungal inoculum and enhanced by NaCl presence. In the assay conditions, H. burtonii and C. zeylanoides were the most effective, both in inhibiting P. nordicum growth and OTA production. D. hansenii was the species with the lowest inhibitory activity, especially in the absence of salt. OTA production dropped from the range < LOD - 5000 ppb in P. nordicum control plates to the range < LOD - 200 ppb in yeast-added plates. OTA production increased in the presence of NaCl in P. nordicum control plates, while salt enhanced inhibition against OTA production in yeast-added plates.

Characteristics of spoilage-associated secondary cucumber fermentation

Secondary fermentations during the bulk storage of fermented cucumbers can result in spoilage that causes a total loss of the fermented product, at an estimated cost of $6,000 to $15,000 per affected tank. Previous research has suggested that such fermentations are the result of microbiological utilization of lactic acid and the formation of acetic, butyric, and propionic acids. The objectives of this study were to characterize the chemical and environmental conditions associated with secondary cucumber fermentations and to isolate and characterize potential causative microorganisms. Both commercial spoilage samples and laboratory-reproduced secondary fermentations were evaluated. Potential causative agents were isolated based on morphological characteristics. Two yeasts, Pichia manshurica and Issatchenkia occidentalis, were identified and detected most commonly concomitantly with lactic acid utilization. In the presence of oxygen, yeast metabolic activities lead to lactic acid degradation, a small decline in the redox potential (E(h), Ag/AgCl, 3 M KCl) of the fermentation brines, and an increase in pH to levels at which bacteria other than the lactic acid bacteria responsible for the primary fermentation can grow and produce acetic, butyric, and propionic acids. Inhibition of these yeasts by allyl isothiocyanate (AITC) resulted in stabilization of the fermented medium, while the absence of the preservative resulted in the disappearance of lactic and acetic acids in a model system. Additionally, three Gram-positive bacteria, Lactobacillus buchneri, a Clostridium sp., and Pediococcus ethanolidurans, were identified as potentially relevant to different stages of the secondary fermentation. The unique opportunity to study commercial spoilage samples generated a better understanding of the microbiota and environmental conditions associated with secondary cucumber fermentations.

PMKT2, a new killer toxin from Pichia membranifaciens, and its promising biotechnological properties for control of the spoilage yeast Brettanomyces bruxellensis

Pichia membranifaciens CYC 1086 secretes a killer toxin (PMKT2) that is inhibitory to a variety of spoilage yeasts and fungi of agronomical interest. The killer toxin in the culture supernatant was concentrated by ultrafiltration and purified to homogeneity by two successive steps, including native electrophoresis and HPLC gel filtration. Biochemical characterization of the toxin showed it to be a protein with an apparent molecular mass of 30 kDa and an isoelectric point of 3.7. At pH 4.5, optimal killer activity was observed at temperatures up to 20 degrees C. Above approximately this pH, activity decreased sharply and was barely noticeable at pH 6. The toxin concentrations present in the supernatant during optimal production conditions exerted a fungicidal effect on a variety of fungal and yeast strains. The results obtained suggest that PMKT2 has different physico-chemical properties from PMKT as well as different potential uses in the biocontrol of spoilage yeasts. PMKT2 was able to inhibit Brettanomyces bruxellensis while Saccharomyces cerevisiae was fully resistant, indicating that PMKT2 could be used in wine fermentations to avoid the development of the spoilage yeast without deleterious effects on the fermentative strain. In small-scale fermentations, PMKT2, as well as P. membranifaciens CYC 1086, was able to inhibit B. bruxellensis, verifying the biocontrol activity of PMKT2 in simulated winemaking conditions.

Unusual properties of the halotolerant yeast Candida nodaensis Killer toxin, CnKT

CnKT, the Killer toxin from the extreme halotolerant yeast Candida nodaensis, presents a strong salt-stimulated phenotype and is a resilient toxin, able to cope with very diverse and aggressive environmental conditions. This zymocin is active in a broad range of pH and temperature and tolerates freezing and conservation for long periods of time. CnKT stability is increased under very high ionic strength and its activity is stimulated by sodium ions, which might interfere in the zymocin structure/stability. All these characteristics make CnKT a promising candidate for several biotechnological applications, e.g. in the high-salt food products preservation from spoilage by other yeasts.

Cwp2p, the plasma membrane receptor for Pichia membranifaciens killer toxin

PMKT is a channel-forming killer toxin secreted by Pichia membranifaciens. To identify novel genes that mediate cellular resistance to PMKT we screened a collection of 288 deletion mutants. We found 29 open reading frames (ORFs) that, when deleted, confer resistance to PMKT. In addition, the deletion of 15 ORFs was observed to increase protoplast resistance, in agreement with the initial assumption that a plasma membrane receptor for PMKT exists. Whole cells and protoplasts of a cwp2Delta mutant were found to be completely resistant to PMKT and were unable to bind PMKT, indicating that Cwp2p interacts with it. A protein with a molecular mass of 11.7 kDa was purified from PMKT-affinity columns. This protein was sequenced and identified as Cwp2p. Glycosylphosphatidylinositol (GPI) anchoring-defective mutants were much less sensitive to PMKT, as were wild-type protoplasts pretreated with phosphatidylinositol-specific phospholipase C to remove GPI-anchored proteins, indicating that the GPI-anchored precursor of Cwp2p is also necessary for PMKT activity. Carboxyfluorescein-entrapped liposomes containing a purified GFP-Cwp2p fusion protein in their membranes were much more sensitive to PMKT than protein-free liposomes. Cwp2p and its GPI-anchored precursor are proposed for the first time to be involved as PMKT secondary receptors.

The use of killer sensitivity patterns for biotyping yeast strains: the state of the art, potentialities and limitations

In recent years molecular techniques have been the most useful tools for the unequivocal identification of undetermined strains at the species level. In many instances, however, a further discrimination at the strain level (biotyping) is required, such as during epidemiological investigations, in which the distribution of pathogenic microorganisms is studied, and for patent protection purposes. Although molecular methods are routinely used also for yeast biotyping, several nonmolecular techniques have been proposed. One of these, the determination of the killer sensitivity pattern (KSP) towards a panel of selected killer toxins has proven to be a good auxiliary method. Despite the plethora of studies published, the potential and limitations of the determination of KSPs have never been critically evaluated. In this review the use of this nonmolecular technique as a biotyping tool is discussed and compared with some currently used DNA-based procedures. In addition, methodological, mechanistic and ecological implications are evaluated.

Debaryomyces hansenii — an extremophilic yeast with biotechnological potential

We illuminate the ecological, physiological and genetic characteristics of the yeast Debaryomyces hansenii in the view of our belief that this metabolically versatile, non-pathogenic, osmotolerant and oleaginous microorganism represents an attractive target for fundamental and applied biotechnological research. To this end, we give a broad overview of extant biotechnological procedures using D. hansenii, e.g. in the manufacture of various foods, and propose research into the heterologous synthesis of a range of fine chemicals.

The transcriptional response of Saccharomyces cerevisiae to Pichia membranifaciens killer toxin

The transcriptional response of Saccharomyces cerevisiae to Pichia membranifaciens killer toxin (PMKT) was investigated. We explored the global gene expression responses of the yeast S. cerevisiae to PMKT using DNA microarrays, real time quantitative PCR, and Northern blot. We identified 146 genes whose expression was significantly altered in response to PMKT in a non-random functional distribution. The majority of induced genes, most of them related to the high osmolarity glycerol (HOG) pathway, were core environmental stress response genes, showing that the coordinated transcriptional response to PMKT is related to changes in ionic homeostasis. Hog1p was observed to be phosphorylated in response to PMKT implicating the HOG signaling pathway. Individually deleted mutants of both up- (99) and down-regulated genes (47) were studied for altered sensitivity; it was observed that the deletion of up-regulated genes generated hypersensitivity (82%) to PMKT. Deletion of down-regulated genes generated wild-type (36%), resistant (47%), and hypersensitive (17%) phenotypes. This is the first study that shows the existence of a transcriptional response to the poisoning effects of a killer toxin.

Yeasts as biological agents to control Botrytis cinerea

Yeasts, isolated from different sources, were identified and tested for inhibition using YMA-MB plates seeded with Botrytis cinerea strains. A total of 42 yeast strains of 20 different species were tested in vitro for antagonism against 18 pathogenic B. cinerea strains. Pichia membranifaciens, P. anomala and Debaryomyces hansenii displayed the most important inhibitory effect against Botrytis strains. In small-scale trials, post-harvest application of P. membranifaciens CYC 1106 to apple wounds inhibited B. cinerea CYC 20010. Purified killer toxin from P. membronifaciens CYC 1106 inhibited B. cinerea CYC 20010. Results indicated that certain yeasts, or their toxins such us P. membranifaciens CYC 1106 killer toxin, might have potential as novel agents to control B. cinerea.

Factors affecting killer activity of some yeast species occurring in rokpol cheese

Fourteen strains of Candida famata and 7 strains of C. sphaerica originating from blue-veined Rokpol cheese were studied for their ability to produce killer toxins against 3 strains of Yarrowia lipolytica selected as potential starter cultures for cheesemaking. All tested strains revealed killer activity in the presence of salt, at 14 degrees C and pH 4.6. Only one Y. lipolytica strain was resistant to C. famata and C. sphaerica killer toxins.

Killer toxin of Pichia membranifaciens and its possible use as a biocontrol agent against grey mould disease of grapevine

The use of Pichia membranifaciens CYC 1106 killer toxin against Botrytis cinerea was investigated. This strain exerted a broad-specificity killing action against other yeasts and fungi. At pH 4, optimal killer activity was observed at temperatures up to 20 degrees C. At 25 degrees C the toxic effect was reduced to 70%. The killer activity was higher in acidic medium. Above about pH 4.5 activity decreased sharply and was barely noticeable at pH 6. The killer toxin protein from P. membranifaciens CYC 1106 was purified to electrophoretic homogeneity. SDS-PAGE of the purified killer protein indicated an apparent molecular mass of 18 kDa. Killer toxin production was stimulated in the presence of non-ionic detergents. The toxin concentrations present in the supernatant during optimal production conditions exerted a fungicidal effect on a strain of B. cinerea. The symptoms of infection and grey mould observed in Vitis vinifera plants treated with B. cinerea were prevented in the presence of purified P. membranifaciens killer toxin. The results obtained suggest that P. membranifaciens CYC 1106 killer toxin is of potential use in the biocontrol of B. cinerea.

Production and characteristics of Debaryomyces hansenii killer toxin

The optimal conditions for the production of the killer toxin of Debaryomyces hansenii CYC 1021 have been studied. The lethal activity of the killer toxin increased with the presence of NaCl in the medium used for testing the killing action. Production of the killer toxin was stimulated in the presence of proteins of complex culture media. Addition of nonionic detergents and other additives, such as dimethylsulfoxide enhanced killer toxin production significantly. Killer toxin secretion pattern followed the growth curve and reached its maximum activity at the early stationary phase. Optimal stability was observed at pH 4.5 and temperatures up to 20 degrees C. Above pH 4.5 a steep decrease of the stability was noted. The activity was hardly detectable at pH 5.1.

(1-->6)-Beta-D-glucan as the cell wall binding site for Debaryomyces hansenii killer toxin

AIMS

The aims of this study were to characterize the cell wall binding site of Debaryomyces hansenii killer toxin to provide a simple purification method and to determine some characteristics of this toxin.

METHODS AND RESULTS

Various linear (1-->6)-beta-D-glucans of different origins were effective competitive inhibitors of the toxin action. Periodate oxidation and 1H-NMR was used to determine the receptor nature. Affinity chromatography on pustulan-Sepharose column was used to purify D. hansenii killer toxin, probably a 23-kDa protein. The killer toxin character was cureless.

CONCLUSIONS

The investigation revealed that the killer toxin was mainly adsorbed by (1-->6)-beta-D-glucans. This is a low molecular weight protein, probably encoded by chromosomal genes.

SIGNIFICANCE AND IMPACT OF THE STUDY

The specificity of the killer toxin for its receptor provides an effective means to purify the killer toxin. This study is the first to identify the cell wall binding site of this killer toxin, a toxin with properties of industrial relevance.

The growth, properties and interactions of yeasts and bacteria associated with the maturation of Camembert and blue-veined cheeses

The growth of yeasts and bacteria were monitored during the maturation of Camembert and blue-veined cheese produced in Australia. Yeasts were prominent throughout maturation, growing to 10(5)-10(9)/g, depending on the manufacturer. Debaryomyces hansenii predominated, but there were lesser, inconsistent contributions from Yarrowia lipolytica. Of the non-lactic acid bacteria, Acinetobacter species were significant during the maturation of Camembert but not blue-veined cheeses, and grew to 10(6)-10(8) cfu/g. Staphylococcus and Micrococcus species were consistently isolated from the cheeses with Staphylococcus xylosus growing to 10(5)-10(9) cfu/g, depending on the product. Lactic acid bacteria (10(7)-10(9) cfu/g) were present throughout maturation but were not identified. Interactions between the various yeasts and bacterial isolates were examined. Several strains of D. hansenii exhibited killer activity but not against Y. lipolytica. None of the yeasts were antagonistic towards the bacteria but some strains of D. hansenii enhanced the growth of Y. lipolytica and S. xylosus. The yeast and bacterial isolates exhibited various degrees of extracellular proteolytic and lipolytic activities.

Debaryomyces hansenii growth in nonsterile seawater ClO2-peptone-containing medium

We found that the marine yeast Debaryomyces hansenii strain C-11 (CIBNOR yeast collection, La Paz, Mexico) is highly tolerant to chlorine dioxide (ClO2), a powerful biocide agent. A direct application of this observation is the fermentation of the yeast in a nonsterile medium with an initial concentration of 0.3 mg/L of ClO2. The disinfectant helps to avoid the growth of unwanted microorganisms while allowing the development of the yeast. Because the concentration of ClO2 decreases during the fermentation, we ascribe to D. hansenii cells a "biocontrol" action that contributes to the collection of a contaminant-free yeast cell biomass.

(1-->6)-beta-D-glucan as cell wall receptor for Pichia membranifaciens killer toxin

The killer toxin from Pichia membranifaciens CYC 1106, a yeast isolated from fermenting olive brines, binds primarily to the (1-->6)-beta-D-glucan of the cell wall of a sensitive yeast (Candida boidinii IGC 3430). The (1-->6)-beta-D-glucan was purified from cell walls of C. boidinii by alkali and hot-acetic acid extraction, a procedure which solubilizes glucans. The major fraction of receptor activity remained with the alkali-insoluble (1-->6)-beta- and (1-->3)-beta-D-glucans. The chemical (gas-liquid chromatography) and structural (periodate oxidation, infrared spectroscopy, and (1)H nuclear magnetic resonance) analyses of the fractions obtained showed that (1-->6)-beta-D-glucan was a receptor. Adsorption of most of the killer toxin to the (1-->6)-beta-D-glucan was complete within 2 min. Killer toxin adsorption to the linear (1-->6)-beta-D-glucan, pustulan, and a glucan from Penicillium allahabadense was observed. Other polysaccharides with different linkages failed to bind the killer toxin. The specificity of the killer toxin for its primary receptor provides an effective means to purify the killer toxin, which may have industrial applications for fermentations in which salt is present as an adjunct, such as olive brines. This toxin shows its maximum killer activity in the presence of NaCl. This report is the first to identify the (1-->6)-beta-D-glucan as a receptor for this novel toxin.