Pneumococcal pneumonia

Pneumococcal pneumonia is a lung infection caused by one specific bacterium, Streptococcus pneumoniae, known as pneumococcus. Under the microscope it is recognized by its lancet shape, somewhat pointed at the ends, and by the fact that it almost always appears in pairs or in short chains. It is gram-positive, the name given to the group of bacteria that hold on to a specific stain used to classify them.

It does not usually travel from outside straight into the lung. It normally settles first in the oropharynx and nasopharynx, that is, the throat and the back of the nose, in people who are healthy and feel nothing. That state is called colonization. Pneumonia develops when those colonies are inhaled and travel down into the lower airway, inside the lung.

What makes it hard to clear is its capsule, a sugar coating that surrounds the wall of the bacterium. Granulocytes, the white blood cells that swallow invaders, cannot reach the wall, and without that contact they cannot do their job: phagocytosis. The capsule works like a slippery coat: the phagocyte finds nothing to grab onto. And the severity of the illness comes not only from the bacterium itself but from the scale of the inflammatory response it sets off.