Gram-negative bacteria are seen less frequently: Haemophilus influenzae (J14), Klebsiella pneumoniae (J15.0), Escherichia coli (J15.5), Pseudomonas aeruginosa (J15.1), Bordetella pertussis, and Moraxella catarrhalis are the most common.

Often, bacteria live in parts of the upper respiratory tract and are continuously being inhaled into the alveoli, the cavities deep in the lungs where gas exchange takes place.

This invasion triggers the immune system to respond by sending white blood cells responsible for attacking microorganisms (neutrophils) to the lungs.

The neutrophils, bacteria, and fluid leaked from surrounding blood vessels fill the alveoli and result in impaired oxygen transportation.

In some very rare cases, bacterial pneumonia can form from someone throwing up and then falling asleep on their side causing some stomach fluid to funnel into the lungs leading to this infection.

In the United Kingdom, amoxicillin is used as first-line therapy in the vast majority of patients acquiring pneumonia in the community, sometimes with added clarithromycin.

An extremely sick individual may require artificial ventilation and intensive care as life-saving measures while his or her immune system fights off the infectious cause with the help of antibiotics and other drugs.