Production of capsular material by Streptococcus suis serotype 2 under different growth conditions

The procedure currently used for the production of Streptococcus suis antigen is very long and includes several subcultures. The aim of the present work was to study the in vitro production of capsular material by S. suis serotype 2 after each of these subcultures. The amount of capsular material produced was evaluated by electron microscopy using bacterial cells grown on blood-agar plates and in Todd-Hewitt broth (THB) or THB supplemented with serum. In addition, the production of antibodies in rabbits with antigens produced using different growth conditions was compared. Antigens produced after only three subcultures possessed as much capsular material as cells obtained after the complete procedure and induced a similar antibody response. The use of serum as a supplement to the broth did not assure a higher production of capsule; in addition, antibody titers obtained with antigens produced in THB were as high as those obtained with antigens produced in THB supplemented with serum. We recommend the use of three subcultures in nonsupplemented broth for the production of immunogens. This revised protocol offers two main advantages: it is less time-consuming because of the limited number of subcultures and is also less expensive since nonsupplemented broths are used.