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The application of Fourier Transform Infrared Spectroscopy for characterization of yeasts is growing rapidly. Since it is known that the phenotypic expression of yeast cells depends sensitively on the nutrients that are available in the growth medium, one standardized growth medium is usually used for identification and characterization purposes in order to obtain reproducible FTIR signals. Since our recently developed high-throughput micro-cultivation protocol has the capacity to use more than one standardized growth medium, we wanted to investigate if the parallel use of multiple growth media can improve identification results. For this purpose, five different cultivation media (YP, YPD, YMB, SAB and SD) were used. In total 91 food spoilage yeast strains of 12 different genera were cultivated in different cultivation media and subsequently characterized by FTIR spectroscopy. For spectral identifications, Radial Basis Function-Partial Least Squares (RBF-PLS) was used in combination with cross-model validation where an inner cross-validation loop was used to optimize the model, while in an outer loop an independent test set was kept aside to test the optimized model. Sensitivity and specificity were evaluated for each studied genus class. The results show that the YMB selective medium gave the best discrimination results for 9 of the 12 genera with sensitivity above 90%. Only three genera showed better identification results on other media (Clavispora and Metschnikowia on medium SD, Debaryomyces on medium YPD). We therefore suggest to use the media SD, YPD in combination with the YMB medium for the identification of food spoilage yeasts.

It has been clearly established that phytopathogenic fungi, bacteria, and viruses exert biotic stresses on plants. Much less is known, however, about the interactions between enological species of yeast and their host plants. In a previous study, we described how Saccharomyces cerevisiae, the most common enological yeast, can act as a grapevine (Vitis vinifera L.) pathogen, causing growth retardation or plant death. In the present in vitro study on 11 strains of yeast belonging to different genera, which often occur on the surfaces of vineyard grapes and V. vinifera, a link was found to exist between strain phytopathogenecity and pseudohyphal growth habits and/or endopolygalacturonase activity. The results obtained here are consistent with earlier findings showing that the phytopathogenicity of yeast strains depends on the filamentous growth process, and show that endopolygalacturonase alone is not responsible for the invasion of plants tissues. The mechanisms observed here may be of significant ecological importance and may help to explain the long periods of yeast survival found to occur in vineyards.

Previous work in our laboratory has shown that Saccharomyces bayanus strain SCPP is the only reported yeast expressing the three types of pectolytic enzymes: pectin esterases, pectin lyases and polygalacturonases. One of these enzymes, the endopolygalacturonase (endoPG), hydrolyses plant-speci®c polysaccharide pectin. The endoPG encoding gene (PGU1) is also present in Saccharomyces cerevisiae. It has been shown that this endoPG is required for the development of pseudohyphae. Using genomic DNA, the PGU1-1 and PGU1-2 promoters of these strains have been ampli®ed and used to construct gene fusions with the b-galactosidase gene. On the basis of b-galactosidase measurements, we compared the expression of both promoters in different environmental conditions in order to identify their modulation. We have shown that the PGU1 gene is upregulated by the presence of the pectin and the product resulting from endopolygalacturonase activity. Moreover, expression of the PGU1 is also enhanced under respiratory and ®lament formation conditions.