Phagocytosis prevents the multiplication of Streptococcus suis in blood samples from conventionally reared adult pigs. It is mediated by antibody, probably IgM, present in the serum of adults and specifically directed against the capsular polysaccharide of S. suis. The blood of young pigs reared in a ‘pathogen-free’ environment allows multiplication of encapsulated S. suis. Of 160 piglets conventionally reared, the blood of 92(58%) did not permit multiplication of S. suis. Blood from the remaining 68(42%) allowed multiplication but became bacteriostatic when the animals reached the age of 6 to 8 weeks. Serum from adult pigs confers bacterio static activity on blood from susceptible piglets in vitro and passively protects such piglets against experimental S. suis infection in vivo.
We thank Dr T. J. L. Alexander, formerly of Ontario Veterinary College, and Mr P. C. Trexler of the Royal Veterinary College, London, for providing us with serum from gnotobiotic pigs.
The work described in this report was carried out by one of us (K. K. A.) during the tenure of a Commonwealth Scholarship and was incorporated in a dissertation submitted for the Ph.D. degree of the University of Cambridge.