Bacterial imaging is an emerging technology that has many health and environmental applications. For example, there is a need to develop highly sensitive assays that can detect very small numbers of pathogenic bacterial cells in food, drinking water, or biomedical samples. In other situations, the goal is to study temporal and spatial distribution of bacteria in live animals. Optical imaging of bacteria in vivo is much less developed than methods such as radioimaging and MRI. Optical imaging of bacterial infection in living animals is usually conducted with genetic reporters such as light-emitting enzymes or fluorescent proteins. However, there are many circumstances where genetic reporters are not applicable, and there is a need for exogenous synthetic probes that can selectively target bacteria. Our studies have focused on the development and application of small molecule fluorescent imaging probes that are composed of a bacterial affinity ligand conjugated to a near-infrared fluorophore. The affinity group is a synthetic zinc (II) coordination complex, while the near-infrared fluorophores consist of small organic dyes such as Cyanines or squaraine rotaxanes. Fluorescence microscopy experiments show that our probes rapidly target the anionic surfaces of bacterial cells and have enabled real time fluorescence imaging of such biological events as bacterial binary fission. In mouse intramuscular infection models we have selectively detected and imaged both gram positive and gram negative bacterial infections after intravascular administration of our probes.
Optical images of a mouse with a S. aureus infection in the left rear thigh muscle. Images were acquired before (A), and immediately following (B), intravenous injection of a NIR bacterial imaging probe, and at 6 h (C), 12 h (D), 18 h (E), and 21 h (F). Scale represents the same relative fluorescence intensity for all six images in arbitrary units.
Leevy WM, Gammon ST, Jiang H, Johnson JR, Maxwell DJ, Jackson EN, Marquez M, Piwnica-Worms D, Smith BD. Optical imaging of bacterial infection in living mice using a fluorescent near-infrared molecular probe. J Am Chem Soc 2006; 128(51): 16476-7.