Human Serum Modulates Biofilm Formation By Strains Of Viridans Group Streptococcus Associated With Infective Endocarditis

Oral Streptococcus spp. are among the most frequent bacteria associated with infective endocarditis (IE), forming biofilms on cardiac tissues. This study aimed to evaluate the contribution of serum components to the capacity of strains of oral streptococci associated with IE to form biofilms. To this end, biofilm phenotypes were determined in eight bacterial strains isolated from the bloodstream of patients with IE, which were taxonomically identified using PCR with species-specific primers as Streptococcus sanguinis (n=2), Streptococcus gordonii (n=3) or Streptococcus salivarius (n=2). Reference strains of each species (SK36, Challis, and NCTC 8618) were also tested. The biomass of biofilms formed during 18h was measured in 96-well plates with BHI medium with 1% sucrose (BHIS), supplemented with 10% saliva or 20% human serum. Initial stages of biofilm formation (4 h) in these culture media were also analyzed by scanning electron microcopy (SEM). Supplementation of BHIS with serum significantly increased the biofilm biomass as compared to non-supplemented BHIS in strains of S. sanguinis (2/3, 2.69 versus 1.85), S. gordonii (3/4, 1.41 versus 1.12) and S. salivarius (2/3; 3.36 versus 2.64) (p<0.05). Differently, no significant changes in biofilm formation was promote by BHIS supplementation with saliva. SEM analyzes confirmed the contributing effects of serum in biofilm formation. These data highlights the significant effect of serum in biofilm formation capacity by oral streptococci, further indicating that bacterial interactions with serum components likely contribute to cardiovascular virulence of oral streptococci. Keywords : Streptococcus viridans, biofilm, serum, endocarditis. Highlight : Serum induces biofilm formation of Viridans group Streptococcus.