Noninvasive imaging of Staphylococcus aureus infections with a nuclease-activated probe

Diagnosis of bacterial infections is largely based on nonspecific criteria, with definitive diagnoses made only after biopsy or culture. Frank Hernandez and his colleagues demonstrate noninvasive imaging of Staphylococcus aureus infections in mice with an activatable fluorescent molecular imaging probe. The approach exploits the properties of micrococcal nuclease and uses short, synthetic oligonucleotides that are highly sensitive to micrococcal nuclease but are rendered resistant to mammalian serum nucleases by chemical modifications. Technologies that enable the rapid detection and localization of bacterial infections in living animals could address an unmet need for infectious disease diagnostics. We describe a molecular imaging approach for the specific, noninvasive detection of S. aureus based on the activity of the S. aureus secreted nuclease, micrococcal nuclease (MN). Several short synthetic oligonucleotides, rendered resistant to mammalian serum nucleases by various chemical modifications and flanked with a fluorophore and quencher, were activated upon degradation by purified MN and in S. aureus culture supernatants. A probe consisting of a pair of deoxythymidines flanked by several 2′-O-methyl–modified nucleotides was activated in culture supernatants of S. aureus but not in culture supernatants of several other pathogenic bacteria. Systemic administration of this probe to mice bearing S. aureus muscle infections resulted in probe activation at the infection sites in an MN-dependent manner. This new bacterial imaging approach has potential clinical applicability for infections with S. aureus and several other medically important pathogens.