All viral pneumonia falls under the atypical category as well as three bacteria (Mycoplasma, Legionella, and Chlamydophila)
The main species that causes pneumonia is Legionella pneumophila, has 16 different serogroups. It’s relatively resistant to chlorine, grows at high temperatures, up to 46 degrees C. Grows well in nature inside free-living amoebae. In patients, organisms are mostly intracellular, especially in macrophages. Virulence is related to LPS endotoxin, flagellin. It is slow-growing – culture on charcoal yeast extract takes 3-5 days.
It’s a small gram-negative bacterium, its LPS is truncated and not very endotoxic, peptidoglycan not confirmed. Obligate intracellular organism, requires ATP from host cells for energy needs.