Sialic acid identity modulates host tropism of sialoglycan-binding viridans group streptococci

Microbial interactions with multiple species may expand the range of potential hosts, supporting both pathogen reservoirs and zoonotic spillover. Viridans group streptococci interact with host cells by engaging protein-attached glycosylations capped with terminal sialic acids (sialoglycans). One potential origin for host tropism of these streptococci arises because humans exclusively synthesize the N-acetylneuraminic acid (Neu5Ac) form of sialic acid, while non-human mammals synthesize both Neu5Ac and a hydroxylated N-glycolylneuraminic acid (Neu5Gc). However, the link between binding preference for these sialic acids and preference for the host has not been tested experimentally. Here, we investigate sialoglycan-binding by Neu5Ac/Neu5Gc cross-reactive Siglec-like binding regions (SLBRs) from two strains of streptococci, Streptococcus gordonii strain Challis (SLBR_Hsa) and Streptococcus sanguinis strain SK36 (SLBR_SrpA). Structural and computational analyses of SLBR_Hsa identified molecular details for the binding of disaccharides capped in Neu5Ac or Neu5Gc. Engineering SLBR_Hsa and SLBR_SrpA for narrow selectivity to synthetic Neu5Gc-terminated glycans shifted the binding preference from authentic human plasma receptors to plasma receptors from rat sources. However, host receptor preference did not fully recapitulate purified Neu5Ac/Neu5Gc-capped sialoglycan preference. These findings suggest that sialic acid identity modulates, but does not uniquely determine, host preference by these streptococci. This work refines our understanding of host specificity and challenges prevailing assumptions about the relative role of sialic acids in host tropism.