Visualizing Implanted Tumors in Mice with Magnetic Resonance Imaging Using Magnetotactic Bacteria

Purpose: To determine if magnetotactic bacteria can target tumors in mice and provide positive contrast for visualization using magnetic resonance imaging. Experimental Design: The ability of the magnetotactic bacterium, Magnetospirillum magneticum AMB-1 (referred to from here as AMB-1), to confer positive magnetic resonance imaging contrast was determined in vitro and in vivo. For the latter studies, AMB-1 were injected either i.t. or i.v. Bacterial growth conditions were manipulated to produce small (similar to 25-nm diameter) magnetite particles, which were observed using transmission electron microscopy. Tumor targeting was confirmed using Cu-64-labeled bacteria and positron emission tomography and by determination of viable cell counts recovered from different organs and the tumor. Results: We show that AMB-1 bacteria with small magnetite particles generate T1-weighted positive contrast, enhancing in vivo visualization by magnetic resonance imaging. Following i.v. injection of 64Cu-labeled AMB-1, positron emission tomography imaging revealed increasing colonization of tumors and decreasing infection of organs after 4 hours. Viable cell counts showed that, by day 6, the bacteria had colonized tumors but were cleared completely from other organs. Magnetic resonance imaging showed a 1.22-fold (P = 0.003) increased positive contrast in tumors on day 2 and a 1.39-fold increase (P = 0.0007) on day 6. Conclusion: Magnetotactic bacteria can produce positive magnetic resonance imaging contrast and colonize mouse tumor xenografts, providing a potential tool for improved magnetic resonance imaging visualization in preclinical and translational studies to track cancer. (Clin Cancer Res 2009;15(16):5170-7)