Relationship between intracellular survival in macrophages and pathogenicity of Streptococcus suis type 2 isolates

Naturally-occurring Streptococcus suis type 2 meningitis affects pigs and man; experimental models of the disease have also been established in pigs and mice. A sustained, high-level bacteraemia is an important phase preceding the development of S. suis type 2 meningitis. The main cellular clearance mechanism for circulating bacteria is the resident hepatic and splenic macrophages. The interaction between various isolates of S. suis type 2 and murine macrophages was investigated to determine whether there were differences in the outcome of the interaction that would reflect observed differences in pathogenicity. Phagocytosed non-pathogenic isolates were killed whereas intracellular pathogenic organisms survived and replicated within phagosomes in the absence of anti-S. suis type 2 antibody and complement. The addition of anti-S. suis type 2 antibody and complement to macrophages containing ingested pathogenic organisms resulted in inactivation of the intracellular bacteria. Thus whilst the pathogenicity of S. suis type 2 isolates may be related to an ability to survive within macrophages, immunity to S. suis type 2 meningitis may result from anti-S. suis type 2 antibody preventing pathogenic organisms surviving within macrophages.