Human serum albumin stabilizes streptolysin S activity secreted in the extracellular milieu by streptolysin S-producing streptococci.

Anginosus group streptococci (AGS) are opportunistic pathogens of the human oral cavity; however, their pathogenicity has not been discussed in detail. Oral streptococci live in the gingival sulcus, from where they can easily translocate into the bloodstream due to periodontal diseases and dental treatment and cause hazardous effects on the host through their virulence factors. Streptolysin S (SLS), a pathogenic factor produced by β-hemolytic species/strains belonging to AGS, plays an important role in damaging host cells. Therefore, we investigated the SLS-dependent cytotoxicity of β-hemolytic Streptococcus anginosus subsp. anginosus (SAA), focusing on different growth conditions such as in the bloodstream. Consequently, SLS-dependent hemolytic activity/cytotoxicity in the culture supernatant of β-hemolytic SAA was stabilized by blood components, particularly human serum albumin (HSA). The present study suggests that the secreted SLS, not only from β-hemolytic SAA, but also from other SLS-producing streptococci, is stabilized by HSA. As HSA is the most abundant protein in human plasma, the results of this study provide new insights into the risk of SLS-producing streptococci which can translocate into the bloodstream.