The results showed that the cell surface-displayed phytase was as least as effective as the secreted phytase in hydrolyzation of phytic acid under conditions similar to the digestive tract of chickens. Although phytase has previously been displayed on the cell surface of S. cerevisiae (Mo et al., 2005), its utilization as a feed supplement has never been demonstrated. As the rPhyA170-agg exhibits two peaks of optimal pH at 3 and 5.5 (which are similar to pH ranges in the stomach

and intestine of most animals), along with its stability over a broad pH range from 2 to 8, it is ideal for application as a whole-cell feed supplement without SAHA HDAC in vivo the requirement for downstream purification processing normally associated with secreted phytase. This would save cost and time for the feed industry. Yeast cells harboring cell-surface-displayed phytase were analyzed further for their nutritional contents by proximate analysis (Table

1). When the celPhyA170-agg cells were added to feedstuff (at 6% w/w), the biotin content was significantly increased by approximately 68% compared with the control feedstuff. In addition, with the addition of yeast cells, niacin content was also increased by approximately 12%. Yeasts, especially S. cerevisiae, have long been used as feed supplements because of their many potential advantages. For example, Zhang et al. (2005) found that S. cerevisiae cell components added to broiler chicks could improve growth performance and meat tenderness in addition to better feed/gain ratio and body weight gain compared with control TGF-beta Smad signaling feed without yeasts (Zhang et al., 2005). Yeast cells harboring cell-surface phytase and containing biotin, niacin, and proteins can, thus, potentially enhance the growth of animals. Supplement of yeast to feedstuff can also reduce amounts of some ingredients of the feed. For example, whole yeast rich in protein can replace soybean

meal, and yeast cell wall rich in carbohydrates can replace corn to some extent (Zhang et al., 2005). Furthermore, yeast cells potentially contain other vitamins and trace elements, and supplementation of these yeasts to feedstuff can reduce the requirement for these elements, thus lowering cost for the feed industry. Yeast cells containing cell wall mannan oligosaccharides were also reported to enhance immune response against infections (Zhang et al., 2005; Eicher et al., 2006; Santin et al., 2006). In addition to phytase, other polysaccharide- and nonpolysaccharide-degrading enzymes (such as xylanase, cellulase, and protease) are also typically added to feedstuff. Thus, P. pastoris codisplaying phytase with other enzymes on its surface could allow two or more enzymes to be expressed by the same yeast cells and would offer further advantages as a feed supplement. Currently, yeast codisplaying phytase and xylanase on the cell surface is being developed in our laboratory.