<i>Mycobacterium tuberculosis</i> fluorescent and bioluminescent imaging technologies: addressing the issue of sensitivity

Introduction Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb), is a common cause of death in humans worldwide. The slow growth rate of Mtb (~20 hours) makes progress in research slow and diagnosis difficult. Areas covered Imaging technologies that can quantify viability and infectious load can have a profound impact on progress toward new therapeutics and vaccines and allow rapid diagnosis. Although imaging can quantify bacterial loads throughout an entire animal in real-time, sensitivity is a key limitation. Fluorescent and bioluminescent recombinant strains have been used within the TB field and in other bacteria but have a threshold of ~10 3 bacteria in mice. Reporter enzyme fluorescence (REF) is a new and very sensitive Mtb imaging technology that uses BlaC, a highly specific surface-localized β-lactamase, in combination with fluorogenic or bioluminescent probes. As such, REF can reduce the threshold to 10-100 bacteria in vivo and detect 10 bacteria within 10 minutes in vitro. Expert opinion Recombinant reporter systems for imaging bacteria should continue to improve and may reach similar thresholds, but at present, REF remains the most sensitive approach. Furthermore, new more sensitive probes can be readily developed, suggesting that REF will ultimately allow detection of single bacteria in an infected host.