False-colour transmission electron micrograph (TEM) of the Gram-positive bacterium Streptococcus sp.; the elongated cell at top is in the process of division (replication). Streptococci secrete a viscous material to form capsules (red) around their cell walls. The role of the capsule is uncertain, but is associated with virulence in pathogenic organisms, providing protection against the host's immune system. Capsulated organisms resist phagocytosis (engulfing) by scavenging macrophages and are thus able to continue to multiply. Magnification x11,400 at 6x7cm size.

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