Tahia Benitez
Departamento de Genetica, Facultad de Biologia, Universidad de
Sevilla, Sevilla (Spain)


The contribution of the microorganisms to the overall genetic pool is enormous: there are authors who assume that each species of arthropod or vascular plant supports at least one species of nematode, one of protozoon, one of bacterium and one of virus as parasites; it has also been told that the proportion of fungal species with regards to vascular ones is six to one; even that the consequence of ten-fold reduction in length implies that the number of species which fits in such range of size increases one hundred-fold.

The diversity observed in natural populations of microorganisms is considerably higher than that known in the usual laboratory cultures. Among bacteria, there are species able to fix N2 or CO2, oxidise sulphides or methane, reduce CO2 to methane, metabolise xenobiotic compounds or digest macromolecules with extracellular enzymes; other species are parasites of animals or plant pathogens or symbionts; some others are able to grow at very high salt concentrations, pHs lower than one or higher than twelve, temperatures higher than 110:C or in the presence of ionising radiations of high intensity. The genetic diversity within the same species is extremely high in the streptomycetes, and reaches its maximal values in some groups of halobacteria where the probability of a cell to be genetically identical to its parent is of only 80%.

Similarly, yeasts can be isolated from soils from the Tropics to the Polar Regions, from salted or fresh waters, from organic debris, mainly vegetal detritus, from the surface of fruits or the cuticle of insects, from the skin of animals where they live as commensals or pathogens, etc. Yeasts can metabolise an enormous variety of substrates which includes xylose, cellobiose, lactose and other sugars, hydrocarbons and alcohols, xylan, pectic substrates and phenolic compounds. Yeast applications include their use as bakers’yeasts, single cell protein and food production, lipid or ethanol production, or the production of beer, wine, distillery, lactic compounds, glycerine, vitamins, aminoacids, enzymes, polyhydric alcohols and carotenes, among many other compounds.

Since not all yeast strains are able to mate, the classification of yeasts into species has been carried out according to certain physiological features such as the yeast capability to ferment and/or assimilate different substrates. Recently, the interfertility found between strains which had previously been unable to conjugate, has allowed different species to be gathered and classified now as a unique species, as for example it happens with Saccharomyces cerevisiae . However, under an industrial point of view, each of the different species of Saccharomyces which are now classified as Saccharomyces cerevisiae has been associated with a specific fermentation process so that, for practical reasons, the former classifications and names are still being employed.

Together with morphological and metabolical tests, techniques of molecular biology are recently used as a good complement to the classical techniques to classify yeasts. These techniques include electrophoresis of extracellular fractions, studies on the protein profiles, profiles of fatty acids of long chain, polymorphism of DNA sequences, chromosome electrophoresis and analysis of the restriction maps of mitochondrial DNA. These techniques allow to distinguish strains at inter- or intraspecific levels, and therefore to differentiate those strains of the same species which have been isolated from the same ecological environment. The knowledge about the genetic diversity within the same species allows us to guess how far a species threatened with extinction is or is not in the non-return way: when a species is about to be extinct, it has already lost most of its genetic diversity. On the other hand, this diversity has led in many cases to the formation of strains with improved features after constructing hybrids between non- isogenic parental strains.

The application of, first, techniques of molecular biology to establish phylogenetic relationships existing among yeasts; second, the exploitation of the intraspecific variability existing in natural population; and third, the construction of hybrids between non-isogenic strains have been successfully applied to wine yeasts isolated from different Spanish regions. With regards to the first case, phylogenetic relationships among wine yeasts isolated from regions as different as Rioja, La Mancha, Alicante, Jerez or Albariqo have been established, or autoctonous yeasts from musts from Majorca, Canary Islands or Galicia have been characterised. In relation to the second point, the contribution of each of these yeasts to the flavour, organoleptic and analytical features of the different wines has also been established. Finally, with regards to the third point, the increase in ethanol tolerance with respect to their parents, of those hybrids formed between non-isogenic parental strains, isolated from different Spanish musts and already highly ethanol-tolerant has also been established. From these hybrids, there has been carried out a selection in continuous culture controlled by pH of those hybrids able to tolerate and produce the highest ethanol concentrations, in order to be used in ethanol production as an energy source from appropriate substrates as the carbon source.



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4. J.Jimenez and T.Benitez. 1988. Selection of ethanol-tolerant yeast hybrids in pH-regulated continuous culture. Appl. Environ. Microbiol. 54: 917.