Streptococcus suis Enolase Functions as a Protective Antigen Displayed on the Bacterial Cell Surface

Background. Streptococcus suis serotype 2 (SS2) has evolved into a highly infectious entity, posing a great threat to public health. Screening for and identification of protective antigens plays an important role in developing therapies against SS2 infections. Methods. Multiple strategies were used to investigate a new surface protein that has the potential to be a protective antigen. These strategies included molecular cloning, biochemical and biophysical analyses, enzymatic assay, immunological approaches (eg, immunoelectron microscopy), and experimental infections of animals. Results. We identified an enolase gene from SS2 and systematically characterized its protein product, enolase. Biophysical data indicated that S. suis enolase is an octameric protein. Enzymatic assays verified its ability to catalyze the dehydration of 2-phospho-D-glycerate to phosphoenolpyruvate. In consideration of the strong antigenicity of enolase, an efficient enolase-based method was established for monitoring SS2 infections. Combined evidence strongly indicated that SS2 enolase can localize on the bacterial cell surface and facilitate bacterial adherence. Additionally, we found that enolase can confer complete protection against SS2 infection to mice, which suggests that enolase has potential as a vaccine candidate. Conclusions. We conclude that S. suis enolase functions as a protective antigen displayed on the bacterial cell surface and that it can be used to develop new strategies to combat SS2 infections.