Streptococcus oralis subsp. dentisani Produces Monolateral Serine-Rich Repeat Protein Fibrils, One of Which Contributes to Saliva Binding via Sialic Acid

Abstract

Our studies reveal that the oral colonizer and cause of infective endocarditis Streptococcus oralis subsp. dentisani displays a striking monolateral distribution of surface fibrils. Furthermore, our data suggest that these fibrils impact the structure of adherent bacterial chains. Mutagenesis studies indicate that these fibrils are dependent on three serine-rich repeat proteins (SRRPs), here named fibril-associated protein A (FapA), FapB, and FapC, and that each SRRP forms a different fibril with a distinct distribution. SRRPs are a family of bacterial adhesins that have diverse roles in adhesion and that can bind to different receptors through modular nonrepeat region domains. Amino acid sequence and predicted structural similarity searches using the nonrepeat regions suggested that FapA may contribute to interspecies interactions, that FapA and FapB may contribute to intraspecies interactions, and that FapC may contribute to sialic acid binding. We demonstrate that a fapC mutant was significantly reduced in binding to saliva. We confirmed a role for FapC in sialic acid binding by demonstrating that the parental strain was significantly reduced in adhesion upon addition of a recombinantly expressed, sialic acid-specific, carbohydrate binding module, while the fapC mutant was not reduced. However, mutation of a residue previously shown to be essential for sialic acid binding did not decrease bacterial adhesion, leaving the precise mechanism of FapC-mediated adhesion to sialic acid to be defined. We also demonstrate that the presence of any one of the SRRPs is sufficient for efficient biofilm formation. Similar structures were observed on all infective endocarditis isolates examined, suggesting that this distribution is a conserved feature of this S. oralis subspecies.

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

“…FIVAR domains are also found in some characterized adhesins like Ebh and Embp from Staphylococcus aureus and Staphylococcus epidermidis , respectively, and are involved in binding to fibronectin [ 38 ]. The FIVAR/CBM region is followed by two predicted Siglec-like and Unique domains that share structural similarity with similar domains found in the sialic acid-binding SRRPs Hsa (99.38 and 99.26% probability, PDB: 6EFC), SrpA (99.21 and 99.15% probability, PDB: 5KIQ) and GspB (99.06 and 98.97% probability, PDB: 3QC5) ( Fig 5A and 5B ); however, most characterized SRRPs have only one Siglec-like and Unique domain [ 15 , 21 , 39 ]. The NRR is followed by 31 DUF1542 domains, which were previously shown to form fibrils when overexpressed [ 40 ].…”

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

“…The FIVAR/CBM region is followed by two predicted Siglec-like and Unique domains that share structural similarity with similar domains found in the sialic acid-binding SRRPs Hsa (99.38 and 99.26% probability, PDB: 6EFC), SrpA (99.21 and 99.15% probability, PDB: 5KIQ) and GspB (99.06 and 98.97% probability, PDB: 3QC5) ( Fig 5A and B); however, most characterized SRRPs only have one Siglec-like and Unique domain [15,21,39].…”

“…RNA isolation was conducted with the RNeasy mini kit (Qiagen) as previously reported [39]. Briefly, the cell pellet of a 1 mL of bacterial culture grown to an OD600 of 0.…”

Tandem sialoglycan-binding modules in a Streptococcus sanguinis serine-rich repeat adhesin create target dependent avidity effects

“…A survey of NCBI GenBank TM identified over a thousand sequences that may belong to this family. Indeed, all sequenced strains of Streptococcus gordonii and S. sanguinis encode serine-rich repeat adhesins ( 3 ), and homologs have been found in strains of Streptococcus oralis and Streptococcus mitis , as well as other oral streptococci ( 2 , 4 – 6 ). One known functional role of these SRR adhesins is to mediate attachment to protein or glycoprotein receptors, which allows for adherence to host tissues.…”

“…S1 ) ( 4 ), in which the FapC BR -binding region exhibits 25% identity and 39% similarity to SK1 BR . This evolutionary relatedness is consistent with evidence that FapC BR may bind sialoglycans and may be important for oral colonization ( 4 ). The functional implications for tandem domains have not been explained in the literature.…”