Granulicatella elegans endocarditis: a diagnostic and therapeutic challenge

A novel sialic acid-binding adhesin present in multiple species contributes to the pathogenesis of Infective endocarditis

Bacterial binding to platelets is a key step in the development of infective endocarditis (IE). Sialic acid, a common terminal carbohydrate on host glycans, is the major receptor for streptococci on platelets. So far, all defined interactions between streptococci and sialic acid on platelets are mediated by serine-rich repeat proteins (SRRPs). However, we identified Streptococcus oralis subsp. oralis IE-isolates that bind sialic acid but lack SRRPs. In addition to binding sialic acid, some SRRP^- isolates also bind the cryptic receptor β-1,4-linked galactose through a yet unknown mechanism. Using comparative genomics, we identified a novel sialic acid-binding adhesin, here named AsaA (associated with sialic acid adhesion A), present in IE-isolates lacking SRRPs. We demonstrated that S. oralis subsp. oralis AsaA is required for binding to platelets in a sialic acid-dependent manner. AsaA comprises a non-repeat region (NRR), consisting of a FIVAR/CBM and two Siglec-like and Unique domains, followed by 31 DUF1542 domains. When recombinantly expressed, Siglec-like and Unique domains competitively inhibited binding of S. oralis subsp. oralis and directly interacted with sialic acid on platelets. We further demonstrated that AsaA impacts the pathogenesis of S. oralis subsp. oralis in a rabbit model of IE. Additionally, we found AsaA orthologues in other IE-causing species and demonstrated that the NRR of AsaA from Gemella haemolysans blocked binding of S. oralis subsp. oralis, suggesting that AsaA contributes to the pathogenesis of multiple IE-causing species. Finally, our findings provide evidence that sialic acid is a key factor for bacterial-platelets interactions in a broader range of species than previously appreciated, highlighting its potential as a therapeutic target.

Infective endocarditis (IE) is typically a bacterial infection of the heart valves that causes high mortality. Infective endocarditis can affect people with preexisting lesions on their heart valves (Subacute IE). These lesions contain platelets and other host factors to which bacteria can bind. Growth of bacteria and accumulation of host factors results in heart failure. Therefore, the ability of bacteria to bind platelets is key to the development of IE. Here, we identified a novel bacterial protein, AsaA, which helps bacteria bind to platelets and contributes to the development of disease. Although this virulence factor was characterized in Streptococcus oralis, a leading cause of IE, we demonstrated that AsaA is also present in several other IE-causing bacterial species and is likely relevant to their ability to cause disease. We showed that AsaA binds to sialic acid, a terminal sugar present on platelets, thereby demonstrating that sialic acid serves as a receptor for a wider range of IE-causing bacteria than previously appreciated, highlighting its potential as a therapeutic target.

Infective endocarditis due to Granulicatella elegans presenting with musculoskeletal symptoms

A 62-year-old man with essential hypertension and right L4-L5 hemilaminectomy was referred to rheumatology for evaluation of severe arthralgia and myalgia for 12 months. Review of symptoms was significant for night sweats and 20 pounds unintentional weight loss. Physical examination was significant for holosystolic murmur best heard at the cardiac apex of unclear chronicity. Laboratory investigations revealed elevated inflammatory markers, white blood cell count and B-type natriuretic peptide. Transoesophageal echocardiogram showed flail posterior mitral leaflet with severe mitral regurgitation and two vegetations (2.5×1 cm and 1.6×0.3 cm). Abdominal CT showed new focal splenic infarcts, and a brain MRI revealed subacute infarcts, consistent with the embolic phenomenon. Blood cultures grew Granulicatella elegans The patient underwent mitral valve replacement surgery followed by 6 weeks of parenteral therapy with vancomycin and gentamicin, with full recovery at a 3-month follow-up.