Identification of interbacterial adhesion between oral Schaalia species and streptococcal serine-rich repeat glycoproteins

Interbacterial adhesion is central to multispecies biofilm formation. Streptococcal serine-rich repeat glycoproteins (SRRPs) play a well-defined role in oral colonization, yet SRRP involvement in interbacterial adhesion remains understudied. Here, we aimed to isolate SRRP-dependent adhesion partners of Streptococcus gordonii Challis (DL1) from human oral microbiota. Of 179 isolates screened, 23 coaggregated with DL1 but not with an isogenic SRRP-encoding gene deletion mutant. Whole-genome sequencing identified isolates as 19 Schaalia odontolytica, 3 Schaalia sp. HMT-172, and 1 non-Schaalia mixed-species isolate. Each Schaalia isolate also coaggregated with other oral streptococcal species that encoded SRRPs. When Sch. spp. whole cells were probed with a recombinant DL1 SRRP binding domain, which binds to sialic acids, no binding was observed. Formaldehyde and heat treatment of Sch. spp. cells to denature surface proteins reduced coaggregation with DL1 cells, whereas the same treatment of DL1 did not affect coaggregation with Sch. spp. Proteinase K treatment of DL1 or Sch. spp. reduced or eliminated coaggregation, respectively. Based on these observations, we propose that Sch. odontolytica and Sch. sp. HMT-172 surface proteins bind to formaldehyde- and heat-stable epitopes of streptococcal SRRPs, potentially constituting an important interbacterial adhesion interaction in oral biofilm formation.