Identification of oligopeptide-binding protein (OppA) and its role in the virulence of Streptococcus suis serotype 2

The oligopeptide permease (Opp) cassette, an oligopeptide transport system belongs to the superfamily of ATP-binding cassette (ABC) transporter, is widely distributed in bacteria, including Streptococcus suis (S. suis). It is encoded by the opp operon containing oppA, oppB, oppC, oppD, and oppF. In addition to the uptake of peptide, the oppA gene also plays an important role in virulence of many pathogens. In this study, an oppA homologue from the highly virulent S. suis serotype 2 (S. suis 2) strain 05ZYH33 was identified. Flow cytometry and Western blot confirmed that OppA is a surface immunogenic protein and is expressed during S. suis 2 infection. To explore the role of oppA in S. suis 2 growth and pathogenicity, an isogenic 05ZYH33 mutant of oppA (△oppA) was obtained by homologous recombination. Although the complementary strain was not obtained due to the △oppA strain is not transformable, the current data revealed that deletion of the oppA gene in S. suis 2 has greatly affected its growth and virulence. Our data revealed that the growth rate is significantly slow for the △oppA. Adherence of the △oppA strain to human epithelial cells is greatly reduced comparing to the wild strain. Mouse infection experiment showed that inactivation of oppA greatly attenuated the high pathogenicity of S. suis 2. The observed results suggest that OppA is a surface-exposed protein and plays important roles in the growth and pathogenicity of S. suis 2.