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In vitro phagocytosis and survival of Streptococcus suis capsular type 2 inside murine macrophages
C Brazeau1, M Gottschalk1, S Vincelette1
1Groupe de Recherche sur les Maladies Infectieuses du Porc, Faculté de Médecine Vétérinaire, Université de Montréal, Saint-Hyacinthe, Québec J2S 7C6, Canada.
Abstract:
In this study, data on phagocytosis of Streptococcus suis and its survival inside macrophages are presented. Mouse peritoneal macrophages were incubated in the presence of one of five different strains of S. suis capsular type 2: a virulent wild-type strain (1591), a non-capsulated non-virulent mutant strain (M2), a poorly capsulated non-virulent mutant strain (M42), a non-virulent capsulated strain (1330), and the wild-type reference (virulent) strain S735. Opsonized or non-opsonized bacteria were incubated with macrophages in vitro and samples were obtained after 1 and 3 h incubation. Phagocytosis as well as live and dead intracellular organisms were determined by acridine orange and crystal violet staining. After 1 h incubation, non-opsonized virulent and non-virulent capsulated bacteria were poorly phagocytosed (by less than 7% of the macrophages), whereas the non-capsulated non-virulent mutant strain was highly phagocytosed (by more than 68% of the macrophages). The M42 mutant strain was more phagocytosed than the capsulated strains but less than the non-capsulated M2 mutant strain (35%). In contrast, a higher percentage of live bacteria was observed inside macrophages for the capsulated strains (1591 and S735) than for the non- or poorly capsulated mutant strains (M2 and M42). Opsonization of bacteria with rabbit serum or heat-inactivated rabbit serum significantly increased phagocytosis. For every opsonized strain, after 3 h incubation, the percentage of live bacteria within macrophages was considerably lower than the corresponding non-opsonized strains. In conclusion, the capsule of S. suis type 2 appears to act as an important anti-phagocytic factor. However, virulent capsulated non-opsonized strains can be phagocytosed by mouse peritoneal macrophages within which they appear to survive for at least 3 h. Serum factors other than complement increase not only phagocytosis but also intracellular killing of S. suis of both capsulated and non-capsulated strains.
Insights
The capsule of Streptococcus suis type 2 hinders phagocytosis by macrophages. However, virulent strains can survive inside macrophages, and serum factors enhance bacterial killing.
Area of Science:
- Immunology
- Microbiology
- Bacterial Pathogenesis
Background:
- Streptococcus suis is a significant pathogen causing meningitis and septicemia in pigs and humans.
- The capsule of S. suis is a key virulence factor, but its role in evading phagocytosis requires further investigation.
Purpose of the Study:
- To investigate the role of the S. suis type 2 capsule in phagocytosis by mouse peritoneal macrophages.
- To determine the survival of different S. suis strains within macrophages and the effect of opsonization.
Main Methods:
- Incubation of mouse peritoneal macrophages with five different S. suis type 2 strains (virulent, non-virulent, capsulated, non-capsulated).
- Assessment of bacterial phagocytosis and intracellular survival using acridine orange and crystal violet staining after 1 and 3 hours.
- Evaluation of the effect of opsonization with rabbit serum on phagocytosis and intracellular killing.
Main Results:
- Non-opsonized virulent and capsulated S. suis strains were poorly phagocytosed (<7%), while the non-capsulated mutant (M2) was highly phagocytosed (>68%).
- Capsulated strains (1591, S735) showed higher intracellular survival than non- or poorly capsulated mutants (M2, M42).
- Opsonization significantly increased phagocytosis and intracellular killing for all strains, with lower survival rates for opsonized bacteria after 3 hours.
Conclusions:
- The S. suis type 2 capsule acts as an anti-phagocytic factor, contributing to virulence.
- Virulent, non-opsonized capsulated strains can be phagocytosed and survive within macrophages for at least 3 hours.
- Serum factors beyond complement enhance both phagocytosis and intracellular killing of S. suis.