Identification and characterization of four proteases produced by Streptococcus suis

Streptococcus suis is an important worldwide swine pathogen. In this study, we investigated the production of proteases by S. suis serotype 2. Proteases were identified and characterized using chromogenic and fluorogenic assays and zymography. An Arg-aminopeptidase with a molecular mass of 55 kDa was found to be both cell-associated and extracellular. Cell-associated chymotrypsin-like and caseinase activities, belonging to the serine- and metalloprotease classes respectively, were also detected. Lastly, a dipeptidyl peptidase IV (DPP IV) with a molecular mass of 70 kDa was detected in both whole cells and culture supernatants of S. suis serotype 2. Arg-aminopeptidase, caseinase and DPP IV activities were detected in all strains of S. suis serotype 2 tested whereas the chymotrypsin-like activity was only detected in European virulent strains of serotype 2. The optimum pH for all four proteases was between 6 and 8, and the optimum temperature ranged from 25 to 42°C. This is the first report on the production of proteases by S. suis. Further investigations will determine the possible contribution of these proteases in the pathogenicity of S. suis serotype 2.