Moreover, the liver metabotype of GF animals indicated other bacterial-related changes. A lower level of GSSG, the oxidized form of the powerful antioxidative compound GSH (Meister and Anderson, 1983) and a higher level of hypotaurine were observed in two GF animals of a total of four (Figure 2C). Despite the restricted numbers of individuals included in this study, it is possible that a subgroup of animals may exist. GSSG represents 1% of the total amount of glutathione in vivo (Deneke and Fanburg, 1989). In this study, GSH was not observed because it is readily oxidized to GSSG by exposure to atmospheric oxygen during sample preparation. Thus, it can be considered that the observed GSSG reflects the whole amount of glutathione in the liver extract. Normally, glutathione, rather than hypotaurine, is the predominant antioxidative molecule in the liver. Furthermore, it has been demonstrated that hypotaurine is also a strong antioxidative compound (Aruoma et al, 1988; Yancey, 2005). The observation of a high level of hypotaurine concomitant with low level of glutathione indicates a perturbation of the cell response to oxidative stress. Thus, for these two individuals, the higher level of hypotaurine may compensate for the lack of glutathione in the liver. It is noteworthy that the low total glutathione content was associated in these two animals with high levels of glycine, which is an essential amino acid for glutathione biosynthesis (Meister and Tate, 1976). Taken together, these observations indicate a perturbed γ-glutamyl cycle activity in the liver of two GF mice and this may be suggestive of altered cysteine metabolism (Meister, 1988). The low level of total glutathione in GF animals may impact on many metabolic pathways as it is also a coenzyme involved in the regulation of protein synthesis and degradation, as well as in the mechanism of immune system and in the prostaglandin metabolism (Meister and Anderson, 1983; DeLeve and Kaplowitz, 1991; Uhlig and Wendel, 1992; Wang and Ballatori, 1998).