Activatable MRI probes for the specific detection of bacteria

Activatable fluorescent probes have been successfully used as molecular tools for biomedical research in the last decades. Fluorescent probes allow the detection of molecular events, providing an extraordinary platform for protein and cellular research. Nevertheless, most of the fluorescent probes reported are susceptible to interferences from endogenous fluorescence (background signal) and limited tissue penetration is expected. These drawbacks prevent the use of fluorescent tracers in the clinical setting. To overcome the limitation of fluorescent probes, we and others have developed activatable magnetic resonance probes. Herein, we report for the first time, an oligonucleotide-based probe with the capability to detect bacteria using magnetic resonance imaging (MRI). The activatable MRI probe consists of a specific oligonucleotide that targets micrococcal nuclease (MN), a nuclease derived from Staphylococcus aureus. The oligonucleotide is flanked by a superparamagnetic iron oxide nanoparticle (SPION) at one end, and by a dendron functionalized with several gadolinium complexes as enhancers, at the other end. Therefore, only upon recognition of the MRI probe by the specific bacteria is the probe activated and the MRI signal can be detected. This approach may be widely applied to detect bacterial infections or other human conditions with the potential to be translated into the clinic as an activatable contrast agent.

Ratiometric Method for Rapid Monitoring of Biological Processes Using Bioresponsive MRI Contrast Agents

Bioresponsive magnetic resonance imaging (MRI) contrast agents hold great potential for noninvasive tracking of essential biological processes. Consequently, a number of MR sensors for several imaging protocols have been developed, attempting to produce the maximal signal difference for a given event. Here we introduce an approach which could substantially improve the detection of physiological events with fast kinetics. We developed a nanosized, calcium-sensitive dendrimeric probe that changes longitudinal and transverse relaxation times with different magnitudes. The change in their ratio is rapidly recorded by means of a balanced steady-state free precession (bSSFP) imaging protocol. The employed methodology results in an almost four times greater signal gain per unit of time as compared to conventional T₁-weighted imaging with small sized contrast agents. Furthermore, it is suitable for high resolution functional MRI at high magnetic fields. This methodology could evolve into a valuable tool for rapid monitoring of various biological events.