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Zygosaccharomyces bailii
''Zygosaccharomyces bailii'' is a species in the genus ''Zygosaccharomyces''. It was initially described as ''Saccharomyces bailii'' by Lindner in 1895,〔Lindner, 1895. Mikroskopische Betriebskontrolle in den Gärungsgewerben mit einer Einführung in die Hefenreinkultur, Infektionslehre und Hefenkunde. P. Parey, Berlin.〕 but in 1983 it was reclassified as ''Zygosaccharomyces bailii'' in the work by Barnett et al.〔Barnett, J.A., Payne, R.W., Yarrow, D., 1983. Yeasts: Characteristics and Identification. Cambridge University Press, Cambridge.〕
Spoilage resulting from growth of the yeast ''Zygosaccharomyces'' is widespread, which has caused significant economic losses to the food industry. Within this genus, ''Z. bailii'' is one of the most troublesome species due to its exceptional tolerance to various stressful conditions.〔James, S.A., Stratford, M., 2003. Spoilage yeasts with emphasis on the genus ''Zygosaccharomyces''. In: Boekhout, T. and Robert, V. (Eds), Yeasts in food - Beneficial and detrimental aspects. Woodhead Publishing Ltd. and CRC Press, Cambridge, pp. 171-191.〕 A wide range of acidic and/or high-sugar products such as fruit concentrates, wine, soft drinks, syrups, ketchup, mayonnaise, pickles, salad dressing, etc., are normally considered to be shelf-stable, i.e. they readily inactivate a broad range of food-borne microorganisms. However, these products are still susceptible to spoilage by ''Z. bailii''.〔Erickson, J.P., McKenna, D.N., 1999. ''Zygosaccharomyces''. In: Robinson, R.K., Batt, C.A., Patel, P.D. (Eds.), Encyclopedia of Food Microbiology, vol. 3. Academic Press, London, pp. 2359-2365.〕〔Thomas, D.S., Davenport, R.R., 1985. ''Zygosaccharomyces bailii'' - a profile of characteristics and spoilage activities. Food Microbiology 2, 157-169.〕
==Morphology and modes of reproduction==
''Z. bailii'' vegetative cells are usually ellipsoid, non-motile and reproduced asexually by multilateral budding, i.e. the buds can arise from various sites on the cells.〔Boekhout, T. and Phaff, H.J., 2003. Yeast biodiversity. In: In: Boekhout, T. and Robert, V. (Eds), Yeasts in food - Beneficial and detrimental aspects. Woodhead Publishing Ltd. and CRC Press, Hamburg, pp. 1-38.〕〔Sutton, B.C., 1999. Overview of classification of the fungi. In: Robinson, R.K., Batt, C.A., Patel, P.D. (Eds.), Encyclopedia of Food Microbiology, vol. 2. Academic Press, London, pp. 860-871.〕 During the budding process, a parent cell produces a bud on its outer surface. As the bud elongates, the parent cell’s nucleus divides and one nucleus migrates into the bud. Cell wall material is filled in the gap between the bud and the parent cell; eventually the bud is separated to form a daughter cell of unequal size.〔Ketchum, P.A., 1988. Microbiology: Concepts and applications. John Wiley and Sons Inc., New York, pp. 379-400.〕〔Tortora, G.J., Funke, B.R., Case, C.L., 1992. Microbiology - An introduction, 4th ed. The Benjamin/Cummings Publishing Company, Redwood, pp. 296-331.〕 ''Z. bailii'' cell size varies within a range of (3.5 - 6.5) x (4.5 - 11.5) µm and the cells exist singly or in pair, rarely in short chain.〔Pitt, J.I., Hocking, A.D., 1997. Fungi and food spoilage, 2nd ed. Blackie Academic and Professional, Cambridge.〕 It has been observed that the doubling time of this yeast is approximately 3 hours at 23 °C in yeast nitrogen base broth containing 20% (w/v) fructose (pH 4.0). In more stressful conditions, this generation time is significantly extended.〔Cole, M.B., Keenan, M.H.J., 1987. A quantitative method for predicting shelf life of soft drinks using a model system. Journal of Industrial Microbiology 2, 59-62.〕
Besides the asexual reproduction mode, under certain conditions (e.g. nutritional stress) ''Z. bailii'' produces sexual spores (ascospores) in a sac called ascus (plural: asci).〔〔 Normally, each ascus contains one to four ascospores, which are generally smooth, thin-walled, spherical or ellipsoidal.〔〔Mossel, D.A.A., Corry, J.E.L., Struijk, C.B., Baird, R.M., 1995. Essentials of the microbiology of foods - A textbook for advanced studies. John Wiley & Sons, Chichester, pp. 44-47.〕 It should be mentioned that the ascospores are rarely observed as it is difficult and may take a long time to induce their formation; besides many yeast strains lose the ability to produce ascospores on repeated sub-cultures in the laboratory.〔 On various nutrient agars, ''Z. bailii'' colonies are smooth, round, convex and white to cream coloured, with a diameter of 2 – 3 mm at 3 – 7 days.〔〔 As the morphology properties of ''Zygosaccharomyces'' are identical to other yeast genera such as ''Saccharomyces'', ''Candida'' and ''Pichia'', it is impossible to differentiate ''Zygosaccharomyces'' from other yeasts or individual species within the genus based on macroscopic and microscopic morphology observations.〔 Therefore, the yeast identification to species level is more dependent on physiological and genetic characteristics than on morphological criteria.〔
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