False colour scanning electron micrograph (SEM) of Streptococcus pneumoniae (pneumonococcus), a spherical, Gram-positive bacterium associated with pneumonia in humans. The bacteria secrete a viscous material to form a capsule around their cell wall. The role of the capsule is uncertain, but is associated with virulence in pathogenic organisms, conveying protection against the immune system of the host. Capsulated organisms resist phagocytosis (engulfing) by white blood cells and may survive digestion by scavengers cells, continuing to multiply normally. Magnification: x8000 at 6x7 cm size, x4000 at 35mm size.

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