Detection and management of acute endocarditis remains a clinical challenge. We continue to lack a means of visualizing bacteria in situ
, and therefore currently rely on blood cultures for diagnosis. However, negative blood cultures do not rule out the presence of S. aureus
in valvular vegetations. Here, we present a molecular imaging strategy capable of detecting small amounts of S. aureus
in endocarditic vegetations. The approach exploits a mechanism used by S. aureus
to evade the host immune system; namely SC’s ability to clot human plasma9
locally and seal itself off from surveiling immune cells. Prothrombin analogs engineered with fluorescent or PET beacons allowed us to specifically detect S. aureus
vegetation formation non-invasively, and to monitor antibiotic therapy in mice with acute endocarditis. Furthermore, we showed that prothrombin localizes to growing vegetations via the bifunctional binding capability of SC, which anchored itself to the vegetation through binding multiple fibrin(ogen) D domains, whilst concurrently snaring circulating prothrombin.
The ability for coagulase-positive S. aureus
to initiate infection under flow conditions, while simultaneously evading the host immune defense, is a remarkable feat that is poorly understood. Our findings demonstrate that SC plays a greater role in the development of vegetations than was previously appreciated15-18
. We investigated the COOH-terminal repeat units of SC, and found that SC uses these 27-amino acid repeats to form a ternary complex with FragD and prothrombin. Binding of multiple fibrin(ogen) D domains to a single SC molecule may provide the necessary avidity for SC to withstand the shear stress exerted on a growing vegetation, given that a SC repeat has a KD
of 36 ± 8 nM.
Early in vegetation development, SC was ubiquitously expressed throughout the vegetation. As the vegetation matured, however, SC expression at the core was lost. Chevalier et al. recently demonstrated that SC expression was directly repressed by the quorum sensing-controlled RNAIII, which (via direct binding to SC mRNA) arrests translation and facilitates degradation19
. Our results are consistent with the notion that quorum sensing is responsible for regulating the SC mRNA signal, which would thus control SC-dependent prothrombin localization in developing vegetations. Improved understanding of the role of SC may provide alternative antibiotic drug targets, for instance disrupting growth of the vegetations and their ability to evade host immunity.
Previously proposed techniques for imaging endocarditis have generally lacked specificity for bacterial infection14,20-22
. The approach reported here focuses on detection of coagulase-positive S. aureus
, the deadliest pathogen responsible for the majority of acute endocarditis cases. While further studies are needed to explore translation in large animals and humans, the use of a pathogen-targeting PET tracer in patients would not only facilitate identification of S. aureus
from ’in vivo
blood cultures’), but could also inform on the site, the bacterial load, and the activity of the infection and thus guide antibiotic and surgical therapy.