Tibor Deák

Identification of Foodborne Yeasts

Improvements in identification procedures for yeasts lag behind recent developments in taxonomy. Sophisticated genetical and biochemical methods cannot be used in routine identification of yeasts. In foods representing specific and often selective ecological niches for yeasts, usually a restricted number of species are present, and for these a simplified identification key has been devised based only on 10 to 15 tests. Yeasts representing 215 species reported to be present in foods are included in a key, and methods of simplified identification are described.

Candida galli sp. nov., a new yeast from poultry

Six strains of an unknown yeast species, phenotypically resembling Yarrowia lipolytica and isolated from chicken breast and chicken liver, were studied. The investigation of their small (18S) and large (26S) subunit rDNA revealed a robust genetic difference between these strains and the type strain of Y. lipolytica. A consistent difference in the physiological properties, suitable for separation of the two taxa, was also found. The description of the new anamorphic yeast species, Candida galli is given.

Some remarks on “a taxonomic key for the genus Saccharomyces” (Vaughan Martini and Martini 1993)

The practicability of Vaughan Martini and Martinis “Taxonomic Key for the Genus Saccharomyces” for the separation of S. bayanus from other Saccharomyces sensu stricto species was studied. It was concluded that the ability to grow in vitamin free medium was not a suitable character for this purpose. A new wild S. bayanus strain, isolated from exudate of Carpinus betulus was also included in this study. This appears to be the third documented strain of that species isolated outside of an artificial fermentation environment.

Udeniomyces pannonicus sp. nov., a ballistoconidium-forming yeast isolated from leaves of plants in Hungary

Fifteen ballistoconidium-forming yeasts, isolated from the leaves of plants in Hungary, showed morphological, physiological and biochemical characteristics similar to those of Udeniomyces pyricola. The identical sequences of internal transcribed spacer regions for selected strains (HY-16T, HY-29, HY-111 and HY-186) indicated that they should be classified as one species. Although a representative strain, HY-16T, showed a closer relationship to Itersonilia perplexans than to known Udeniomyces species in phylogenetic trees constructed using 18S rDNA and the D1/D2 region of the 26S rDNA sequence, this species was placed in the genus Udeniomyces on the basis of its morphological and chemotaxonomic characteristics. Udeniomycespannonicus sp. nov. (type strain HY-16T = JCM 11145T = NCAIM Y 01556T = CBS 9123T) is proposed.

Significance of active fructose transport in the differentiation of the Saccharomyces sensu stricto group

Results of fructose proton symport and nDNA/nDNA reassociation measurements in 58 wine and beer yeast strains belonging to the Saccharomyces sensu stricto group are presented.

Candida novakii, sp. nov. a new anamorphic yeast species of ascomycetous affinity

Two strains of an undescribed species of the genus Candida were isolated from decaying wood of Quercus sp. A description of the new species Candida novakii is given.

Chapter 23:Culture Media for Detecting and Enumerating Yeasts and Moulds

Dilution plating techniques are designed to determine populations of viable fungal, i.e. yeast and mould, propagules per unit weight or volume of food. Direct plating techniques, on the other hand, are designed to assess the internal mycoflora of individual pieces of foods, e.g. seeds, nuts or dried fruits, and results are expressed as a percentage of infected pieces. Both techniques are used by industry and regulatory agencies to monitor fungal contamination at various stages of food handling, storing, processing and marketing. Peptone (0.1%) water is commonly used as a diluent for samples to be homogenized, pummelled or blended. Buffered diluents containing up to 30% glycerol, 40% glucose or 60% sucrose are recommended for enumerating xerophiles. No one medium is satisfactory for detection or enumeration of all yeasts and moulds in all foods. Antibiotic-supplemented media are superior to acidified media for general enumeration of yeasts and moulds. Dichloran rose bengal chloramphenicol agar is most suitable for this purpose. Dichloran 18% glycerol agar performs well for enumerating moderately xerophilic yeasts and moulds. Fastidious xerophiles require media containing high concentrations of sugars and/or sodium chloride. Media have been formulated to detect potentially aflatoxigenic aspergilli and mycotoxigenic strains of penicillia, fusaria and other moulds, but media are needed with increased selectivity and specificity for detecting mycotoxigenic moulds. Ascospores of heat-resistant moulds often require heat treatment prior to plating in order to activate the germination process. The spread-plate technique is strongly preferred over the pour-plate technique for enumerating yeasts and moulds. The recommended incubation temperature is 25°C, but incubation time between plating and counting colonies ranges from 5 days for determination of general populations of mycoflora to 4 weeks or more for fastidious xerophiles. There is a need for new and improved media for selectively isolating various groups, genera, species and/or strains of fungi capable of growing only under specific environmental conditions, e.g. low aw, low pH, low oxygen tension or, in the case of sublethally injured cells, under conditions which facilitate resuscitation. Improved media are needed which accurately detect moulds capable of producing specific mycotoxins in a wide range of food types.