Streptococcus intermedius is associated with endogenous infections leading to abscesses in the oral cavity and deep-seated sites, such as the brain and liver. Histone-like protein (HLP), which binds DNA and RNA molecules, lipoteichoic acid and epithelial cells, is well conserved among bacterial species and induces macrophages to produce IL-1 and TNF-a. Objectives: The aim of this study is to clarify the structure and the localization of the HLP. Methods: We cloned the gene encoding S. intermedius HLP from chromosomal DNA by PCR using the primers based on Streptococcus pyogenes hlp gene and inserted this cloned hlp gene into GST gene fusion vector and transformed into E. coli. The GST fusion recombinant HLP protein (rHLP) was produced in E. coli after induction and purified using glutathione sepharose column application. Results: The encoded HLP of S. intermedius has a 91 amino acids, a predicted molecular mass of 9,603Da, an isoelectric point of 10.21, and high sequence identity (88-92% at DNA level, 79-82% at amino acid level) with HLP of several oral Streptococci (S. gordonii, S. sobrinus, S. pyogenes and S. mutans) and 70% identity with S. aureus at the amino acid level. Moreover, 16-mer peptide was selected as an antigen by hydropathy analysis and 3-D structure prediction, and anti-HLP peptide antibody was prepared from peptide-antigen immunized rabbit serum. This anti-HLP peptide antibody reacted specifically as an approximately 9-kDa band in whole cells of S. intermedius, S. pyogenes and methicillin-resistant Staphylococcus aureus and rHLP, and reacted against the interior of S. intermedius and S. aureus cells and the secretion-like component between bacterial cells by immunoelectron microscopic technique. Conclusions: These findings show that HLP of S. intermedius is secreted from bacterial cells and secreted HLP may contribute to tissue injury at infection site, such as abscess, by promoting monocyte/macrophages to induce inflammatory cytokine production.

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