Provided is a P4 peptide, which contains functional epitopes of the PsaA protein of Streptococcus pneumoniae, and related methods and compositions. A peptide that comprises the amino acid sequence defined in SEQ ID NO:1 (an example of the P4 peptide) is provided. Also provided is a peptide comprising the amino acid sequence defined as VPSLFVDSSVDDRPMKTVSQDTNIPIYAQIFTDS (SEQ ID NO:2). P4 peptide mimetics having a conformational structure identical or similar to the conformation of P4 (e.g., SEQ ID NO: 1 and SEQ ID NO:2) are provided. An antibody that specifically binds to the epitope defined by the disclosed peptides is provided. For example, the antibody can specifically bind to a peptide comprising the sequence set forth in SEQ ID NO: 1 and having the conformation defined by the peptide consisting of SEQ ID NO: 1. An antibody that specifically binds to a peptide comprising the sequence set forth in SEQ ID NO:2 and having the conformation defined by the peptide consisting of SEQ ID NO:2 is also provided. A P4-specific antibody is PsaA-specific since P4 defines an epitope specific for PsaA. A vaccine comprising the peptide of SEQ ID NO: 1 and a pharmaceutical carrier is provided. A vaccine comprising a peptide of SEQ ID NO:2 and a pharmaceutical carrier is also provided. Methods of using the peptides and antibodies of the invention are provided. Diagnostic kits comprising a P4 peptide is also provided.La présente invention a trait à un peptide P4, qui contient des épitopes fonctionnels de la protéine PsaA de Streptococcus pneumoniae, et des procédés et compositions associés. Linvention a trait à un peptide qui comporte la séquence dacides aminés définis dans SEQ ID NO: 1 (un exemple du peptide P4). Linvention a également trait à un peptide comportant la séquence dacides aminés définie par VPSLFVDSSVDDRPMKTVSQDTNIPIYAQIFTDS (SEQ ID NO:2). Linvention a également trait à des mimétiques du peptide P4 ayant une structure conformationnelle identique ou semblable à la confo
