Real-time imaging of intestinal bacterial β-glucuronidase activity by hydrolysis of a fluorescent probe

Intestinal bacterial beta-glucuronidase (beta G) hydrolyzes glucuronidated metabolites to their toxic form in intestines, resulting in intestinal damage. The development of a method to inhibit beta G is thus important but has been limited by the difficulty of directly assessing enzyme activity in live animals. Here, we utilized a fluorescent probe, fluorescein di-beta-D-glucuronide (FDGlcU), to non-invasively image the intestinal bacterial beta G activity in nude mice. In vitro cell-based assays showed that the detection limit is 104 colony-forming units/well of beta G-expressing bacteria, and that 7.81 ng/mL of FDGlcU is enough to generate significant fluorescent signal. In whole-body optical images of nude mice, the maximum fluorescence signal for beta G activity in intestines was detected 3 hours after gavage with FDGlcU. Following pretreatment with a bacterial beta G inhibitor, the fluorescence signal was significantly reduced in abdomens and excised intestines images. For a 4-day antibiotic treatment to deplete intestinal bacteria, the FDGlcU-based images showed that the beta G activity was decreased by 8.5-fold on day 4 and then gradually increased after treatment stopped. The results suggested that FDGlcU-based imaging revealed the in vitro and in vivo activity of intestinal bacterial beta G, which would facilitate pharmacodynamic studies of specific bacterial beta G inhibitors in animal studies.