In addition to advances in fluorescent proteins derived from GFP, a new class of fluorescent proteins has recently been isolated

and proven useful in environments deprived VX809 of oxygen. Drepper et al. (2007) demonstrated that FbFPs expressed in the facultative anaerobe Rhodobacter capsulatum in hypoxia was fully fluorescent. More recently, Drepper et al. (2010) have quantitatively monitored the fluorescent intensity of FpFP in vivo in E. coli under oxygen limitations by continuously measuring wavelength excitation at 460 nm and emission at 492 nm. A different study showed that FbFPs expressed in Candida albicans and Saccharomyces cerevisiae under anaerobic conditions render the cells fluorescent (Tielker et al., 2009). In this work, we demonstrate that FbFPs expressed in the obligate anaerobe B. fragilis confer fluorescence to the cells when grown under anaerobic conditions. In the absence of oxygen, B. fragilis cells were remarkably fluorescent (Fig. 2), presenting an emission in the range of 475–505 nm when excited with light at 450 nm in agreement with previous works using FbFPs as the fluorescent marker (Drepper et al., 2007; Tielker et al., 2009). Although GFP protein derivatives have been engineered to increase photostability, intensity, broad

pH range tolerance, faster maturation rates and different colors, which allow researchers to use multiple probes within the same image experimental set (Shaner et al., 2007), Epigenetics Compound Library mouse they are still dependent on molecular oxygen to Ribonucleotide reductase display their fluorescence. This requirement for oxygen for proper post-translational modification of the protein fluorophore is a significant limitation to their use in anaerobic environments. Thus, our findings are important for the study of anaerobic bacteria as there is a lack of imaging tools to study molecular trafficking and gene expression in these organisms, which require anaerobic conditions during their growth and metabolism under both in vitro and in vivo conditions. This is particularly relevant with regard to B. fragilis because it will allow us to investigate this opportunistic

anaerobic human pathogen under low or limited oxygen conditions similar to the ones that occur during anaerobic infection in human tissues. In this regard, as a first step to understand gene expression in B. fragilis during infection, we demonstrate in this study that the ahpC and dps genes are expressed following incubation with a cell line macrophage. These findings indicate that B. fragilis cells were internalized by macrophages and that its intracellular environment induced B. fragilis oxidative stress response as demonstrated by the upregulation of the ahpC∷bs2 and dps∷bs2 transcription fusion. In animal models of intraperitoneal infection, the B. fragilis oxidative stress response is required for survival (Sund et al., 2008).