Characterization of a xylitol producing yeast isolated from corn silage

LOPEZ F; DELGADO O; GONZÁLEZ C; FIGUEROA, L.I.C

Papendal, Arnhem, The Netherlands

Congreso; Tenth International Symposium on Yeast (ISY 2000); 2000

CHARACTERIZATION OF A XYLITOL PRODUCING YEAST ISOLATED FROM CORN SILAGE López F.1.; Delgado O.1, González C.1 and Figueroa, L.I.C. 1, 2 2 Universidad Nacional de Tucumán, Instituto de Microbiología, Cátedra de Microbiología Superior, 4000 Tucumán, Argentina 1 PROIMI ? Av. Belgrano y Pje. Caseros, 4000, Tucumán, Argentina. E-mail: proimiunt@arnet.com.ar Xylitol is a naturally occurring five carbon polyhydroxy alcohol with a high sweetening power. It is increasingly used as food sweetener, dental caries reducer and sugar substitute in the treatment of diabetics. It is a normal intermediate of carbohydrate metabolism in humans and animals. It is also widely distributed in the plant kingdom, particularly in certain fruits and vegetables (da Silva and Afschar, 1994). Xylitol is currently produced by non-specific chemical reduction of D-xylose. Therefore, the biological production of xylitol could be of economical interest since it does not require pure xylose syrup as the chemical synthesis does. Hence, low-cost hemicellulosic hydrolysates might be potential substrates (Roseiro et al., 1991). Procedures commonly used for yeast identification rely on the appearance of cellular morphology and distinctive reactions on a standardized set of fermentation and assimilation test. These tests are laborious and sometimes ambiguous because of strain variability. Given these difficulties and the impracticability for identifying most species from genetic crosses, molecular comparisons are increasingly used for yeast identification (Kurtzman and Robnett, 1998). During the last years interest has turned to another molecular comparisons that include sequencing, restriction fragment length polymorphism, RFLP (Bignell and Evans, 1990; Evans, 1996; Bruns et al., 1991), pulsed field gel electrophoresis (PFGE) and random amplified polymorphic DNA, RAPD (Yuji Oda et al., 1999). Sequencing appears to be the most important technique because strains comparison are easily made and, with the selection of appropriate genes, both close and distant relationships can be resolved (James, et al., 1994; Kurtzman, 1984; Peterson and Kurtzman, 1991; Kurtzman, 1992; Yamada et al., 1996). Partial sequences represent a good and rapid alternative.  McCarroll et al., (1983) and Lane et al., (1985) have reported that phylogenies constructed from portions of the small subunit closely approximate, derived from the complete molecule (Kurtzman and Robnett, 1998). On the other hand non coding sequences such as internal transcribed spacer (ITS) region (James et al., 1996) are an alternative to sequencing of rDNA large and small subunits. The xylitol producing yeast, isolated from corn silage and named in our lab as ASM III, was partially characterized according to Kurtzman and Fell (1998)  using conventional biochemical and physiological studies, as Candida tropicalis. Nevertheless this strain has some characteristics that do not belong to those described for C. tropicalis, for example the yeast ASM III does not grow at 40 °C, it gives positive results to the urea assimilation test, that is, it is urease positive and metabolizes starch. The aim of this work was to characterize systematically a xylitol producing yeast isolated from corn silage by means of both assimilation and fermentation traditional techniques, and molecular methods.