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

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.).

Biotyping of Candida albicans and other fungi by yeast killer toxins sensitivity

Intraspecific differentiation of pathogenic microorganisms is a major need in epidemiological studies concerning the source and spread of infections. This requirement is paramount for those etiologic agents of infectious diseases, which are mainly grouped into one species within the genus, such as Candida albicans. Ideally, laboratory methods for biotyping purposes should be sensitive, reproducible, easy, and economical to perform. In addition, the methods should be flexible in their application to taxonomically unrelated pathogens. We have shown that the toxins produced by a selected panel of killer yeasts, each characterized by a wide spectrum of antimicrobial activity, may be used to discriminate strains belonging to the species of the genus Candida and to other species of eukaryotic and prokaryotic pathogenic microorganisms. The "yeast killer system," which may be sharply increased in sensitivity by addition of further standardized killer yeasts, has proven to be of value in the resolution of many cases of clinical and nosocomial fungal infections. Owing to its reliability, economy, and versatility, this phenotypic system can be used as an alternative biotyping method in laboratories lacking the financial and training resources necessary to perform more sophisticated and expensive molecular approaches.

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.

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.

Assessment of discriminatory power of three different fingerprinting methods based on killer toxin sensitivity for the differentiation of Saccharomyces cerevisiae strains

AIMS

A panel composed of 44 taxonomically certified strains of Saccharomyces cerevisiae of different origin was used to evaluate the discriminatory power of three different fingerprinting methods based on sensitivity towards 24 killer toxins.

METHODS AND RESULTS

Binary data matrix (BDM), triplet data matrix (TDM) and numerical data matrix (NDM) were used as fingerprinting methods. NDM possessed the highest discriminatory power, assessed through the Simpson's, and Hunter and Gaston's indices for the measurement of diversity. The upper limits of fingerprinting ability expressed by the three above methods have been also discussed.

CONCLUSIONS

NDM determined a significant increase of discriminatory power than the use of BDM or TDM, in terms of an effective amplification of their fingerprinting efficacy.

SIGNIFICANCE AND IMPACT OF THE STUDY

The NDM fingerprinting method could find application in control laboratories for the discrimination of yeast strains of industrial importance or covered by patent.

Cryptococcus nemorosus sp. nov. and Cryptococcus perniciosus sp. nov., related to Papiliotrema Sampaio et al. (Tremellales)

Three mycocinogenic strains representing the genus Cryptococcus were isolated on glucuronate agar from plants and turf collected in the Prioksko-terrasny biosphere reserve (Russia). These isolates fit the standard description of Cryptococcus laurentii, but differ from its type strain in both their mycocin-sensitivity profiles and the killing patterns of their mycocins. Sequence analyses of the D1/D2 domain of the 26S rDNA and of the internal transcribed spacer region confirmed that these isolates represent two novel species, for which the names Cryptococcus nemorosus sp. nov. (type strain VKM Y-2906(T)) and Cryptococcus perniciosus sp. nov. (type strain VKM Y-2905(T)) are proposed. Morphological, physiological and biochemical characteristics, as well as mycocinotyping and molecular analysis, show a close affinity between these two novel anamorphic species and the teleomorphic species Papiliotrema bandonii (Tremellales).

Fingerprinting of Yeasts at the Strain Level by Differential Sensitivity Responses to a Panel of Selected Killer Toxins

We used differential sensitivities to a panel of twenty-five cell-free crude killer toxins to fingerprint forty-four Saccharomyces cerevisiae strains of different origin and all taxonomically certified by nDNA-nDNA reassociation. Cluster analysis of numerical data obtained by different growth inhibition areas observed in Petri dishes allowed the complete and reproducible discrimination of all S. cerevisiae strains.

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.