Types of bacteria. Both gram positive and gram negative bacteria can cause pneumonia. Gram-positive bacteria hold on to a bluish-purple color when they are given the Gram stain test, while gram-negative bacteria do not hold on to the stain after they are washed in alcohol. This helps scientists identify them. The most likely bacteria to cause bacterial pneumonia is the gram-positive Streptococcus pneumoniae. It causes pneumonia in every age group save newborns. Others are Bacillus anthracis and Staphylococcus aureus.
Gram-negative bacteria that can cause pneumonia include Klebsiella pneumoniae, E. coli and the bacteria that causes whooping cough. Legionnaire’s disease is a type of pneumonia caused by an atypical bacteria called Legionella pneumophila. Other atypical bacteria that sometimes cause pneumonia are Chlamoydophila pneumoniae and Coxiella burnetii.