Differential killer sensitivity as a tool for fingerprinting wine-yeast strains of Saccharomyces cerevisiae

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.).

Wine yeast molecular typing using a simplified method for simultaneously extracting mtDNA, nuclear DNA and virus dsRNA

Quick and accurate methods are required for the identification of industrial, environmental, and clinical yeast strains. We propose a rapid method for the simultaneous extraction of yeast mtDNA, nuclear DNA, and virus dsRNA. It is simpler, cheaper, and faster than the previously reported methods. It allows one to choose among a broad range of molecular analysis approaches for yeast typing, avoiding the need to use of several different methods for the separate extraction of each nucleic acid type. The application of this method followed by the combined analysis of mtDNA and dsRNA (ScV-M and W) is a highly attractive option for fast and efficient wine yeast typing.

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.

Use of RAPD and killer toxin sensitivity in Saccharomyces cerevisiae strain typing

AIMS

Two different strain characterization techniques, random amplified polymorphic DNA (RAPD) and killer toxin sensitivity (KTS), were compared to assess their typing performance using a set of 30 certified Saccharomyces cerevisiae strains.

METHODS AND RESULTS

A sequential random resampling procedure was employed to subdivide the 32 descriptors in eight sets, in order to compare the differential performances of the two techniques with diverse number of characters. Results showed that RAPD performs better than killer, although the complete differentiation of the strains under study could be obtained only by combining profiles from the two techniques.

CONCLUSIONS

The combination of different typing techniques was useful when discriminating similar organisms. In such cases, the introduction of a second typing technique can be more advantageous than increasing the number of characters obtained with a single method.

SIGNIFICANCE AND IMPACT OF THE STUDY

The distribution of among-strains pairwise distances and the relative performance of the two techniques has implications for the study of biodiversity, taxonomy and microbial ecology.

Combined use of killer biotype and mtDNA-RFLP patterns in a Patagonian wine Saccharomyces cerevisiae diversity study

The aim of this work was to characterize the indigenous wine Saccharomyces cerevisiae diversity within the Argentinean Patagonia. Two cellars with particular enological practices located in different winegrowing areas were selected and 112 indigenous S. cerevisiae isolates were obtained from spontaneous red wine fermentations carried out in them. Thirty-five and 19 patterns were distinguished among the total indigenous isolates using mtDNA-RFLP and killer biotype analysis, respectively. The combination of both typing techniques rendered a higher variability with 42 different patterns, i.e. 42 strains, evidencing a great diversity in S. cerevisiae populations associated with spontaneous red wine fermentations in Northwestern Patagonia. The analysis of the relatedness among strains using Principal Coordinates Analysis from combined data allowed the clustering of the strains into two populations significantly related to their origin fermentations. The combined use of the mtDNA-RFLP analysis together with the killer biotype method proved to be a powerful tool in the fingerprinting of the enological S. cerevisiae strains.

Killer behaviour in wild wine yeasts associated with Merlot and Malbec type musts spontaneously fermented from northwestern Patagonia (Argentina)

The occurrence of killer wine yeasts in Comahue Region (Patagonia, Argentina) was studied. Wild wine yeasts were isolated from spontaneously fermenting Merlot and Malbec type musts. Out of 135 isolates analyzed 37% were sensitive to some well characterized killer toxins as K1-K10 and did not show killer activity (sensitive phenotype, S), 21% showed neutral phenotype (N) and 42% demonstrated killer activity (killer phenotype, K). All but two killer strains, identified as Candida pulcherrima and Kluyveromyces marxianus, were Saccharomyces cerevisiae. Additionally, all killer strains were sensitive to some killer reference strains, showing a killer-sensitive phenotype (KS); neither Saccharomyces or non-Saccharomyces wild yeasts were phenotype killer-resistant (KR). The incidence of the killer character varied with respect to fermentation stage and grape variety, increasing throughout fermentation (13-55% to 36-90%). Irrespective of grape must type, the neutral and sensitive yeasts were ever predominant at initial stages of fermentation. All but six neutral strains, identified as Saccharomyces cerevisiae, were Kloeckera apiculata.

Large-scale screening of selected Candida maltosa, Debaryomyces hansenii and Pichia anomala killer toxin activity against pathogenic yeasts

The killer activity of selected crude toxins produced by nine Candida maltosa, Debaryomyces hansenii and Pichia anomala strains was tested at 37 degrees C against 383 strains belonging to 19 pathogenic species of 10 genera (Candida, Clavispora, Cryptococcus, Filobasidiella, Issatchenkia, Kluyveromyces, Pichia, Saccharomyces, Stephanoascus and Trichosporon). The broad killer spectra exhibited by all crude toxins may lead to the development of new antimycotic agents against medically important yeasts.

Discrimination between Candida albicans and other pathogenic species of the genus Candida by their differential sensitivities to toxins of a panel of killer yeasts

The differential sensitivities to toxins produced by a short panel of four killer yeasts allowed discrimination between 91 strains of the yeast Candida albicans and 223 non-C. albicans Candida strains. One hundred percent of C. albicans isolates exhibited negative results to the toxin panel, while 100% of non-C. albicans cultures gave well-defined and reproducible positive results to at least one of the four killer toxins. Among C. albicans strains only 96 and 87% gave germ tube (GT)- and chlamydospore-positive results, respectively. In addition a few GT-false-positive strains were detected among non-C. albicans isolates. Susceptibility to the toxin panel is apparently expressed more consistently than either GT or chlamydospore production and may constitute a promising basis for a new simple and easy-to-use procedure for routine discrimination between the species C. albicans and other species of the genus Candida.

Differential growth inhibition as a tool to increase the discriminating power of killer toxin sensitivity in fingerprinting of yeasts

A panel of 27 cell-free crude killer toxin preparations were used in fingerprinting 45 Saccharomyces cerevisiae and 11 Saccharomyces exiguus strains. The differential sensitivity to different mycocins was evaluated both as binary data matrix (presence-absence of killing effect), and by considering the growth inhibition areas (measured by agar diffusion well bioassay). The first approach gave an individual fingerprinting of 68% of sensitive strains, whereas the second gave a total and reproducible (P<0.01) discrimination of all tested strains.

Utilisation of differential killer toxin sensitivity patterns for fingerprinting and clustering yeast strains belonging to different genera

The differential killer sensitivity of 103 yeast cultures belonging to 12 species (genera Debaryomyces, Kluyveromyces, Saccharomyces, and Zygosaccharomyces), all previously taxonomically certified by nDNA-nDNA reassociation, against a given panel of 39 killer yeasts was used as a fingerprinting tool. All strains, with the only exception of eight cultures belonging to the species Zygosaccharomyces bailii, were characterised by a specific, individual sensitivity pattern (killer formula). Cluster analysis of binary sequences based on killer sensitivity of strains belonging to different genera is presented and discussed.

The incidence of killer activity of non-Saccharomyces yeasts towards indigenous yeast species of grape must: potential application in wine fermentation

Fourteen killer yeasts were assayed for their ability to kill species of yeast that are commonly associated with fermenting grape must and wine. A total of 147 of a possible 364 killer-sensitive interactions were observed at pH 4.5. Of the killer yeasts studied, Pichia anomala NCYC 434 displayed the broadest killing range. At a pH value comparable with those of wine ferments, pH 3.5, the incidence of killer-sensitive interactions was reduced by 700% across all the yeasts. Williopsis saturnus var. mrakii CBS 1707 exhibited the broadest killing range at the lower pH, killing more than half of the tester strains. Intraspecific variation in sensitivity to killer yeasts was observed in all species where more than one strain was tested. Also, in strains of Pichia anomala, Kluyveromyces lactis and Pichia membranifaciens, the three species in which more than one killer yeast was analysed, intraspecific variation in killer activity was observed.