Lobar pneumonia. Light micrograph of tissue from a lung in a case of lobar pneumonia. Bacterial pneumonia, which results in inflammation of the lung, is most commonly caused by the bacteria Streptococcus pneumoniae. The pathogen is inhaled into a lung segment, or lobe, where it causes the alveoli (air sacs) to become blocked with pus. Here, an interlobar fissure (vertical, centre) separates an area of solidified and damaged lung tissue (left) from an area that is starting to be damaged (right). In the area at right, alveolar structure and air spaces are still seen, unlike the fluid-filled and solidified area at left.