Pneumonia - causes and organisms

1) Gram-Positive Bacteria

    -Streptococcus pneumoniae is the most common cause of pneumonia(20 - 60% of all community-acquired pneumonia-CAP in adults).
    -Staphylococcus aureus causes only about 2% of CAP but 10 - 15% of hospital-acquired pneumonias. It is the organism most often associated with viral influenza.
    -Streptococcus pyogenes or Group A streptococcus.

2) Gram-Negative Bacteria

    These cause infections in patients with chronic lung conditions like cystic fibrosis and chronic lung disease.

    -Haemophilus influenzae
    -Klebsiella pneumoniae may be responsible for pneumonia in alcoholics and other people who are physically debilitated.         

    -Pseudomonas aeruginosa is an important cause of nosocomial pneumonia. It is a common cause of pneumonia.
    -Moraxella catarrhalis, Neisseria meningitidis and E. Coli are other rare gram negative bacteria causing pneumonia.

3) Atypical Pneumonia

    -Mycoplasma pneumoniae is the most common atypical bug causing pneumonia. Mycoplasma is a tiny bacterium that lacks a cell wall. The pneumonia caused by it is commonly called "walking pneumonia."
    -Chlamydia pneumoniae disease is usually mild but can be severe in babies and elderly.
    -Legionella pneumophila causes Legionnaires' disease. It is contracted by breathing in drops of contaminated water (especially from air coolers and air conditioners). Outbreaks are often reported in hotels, cruise ships, and office buildings.
    -Workers exposed to pigeons, parrots, parakeets, and turkeys are at risk for psittacosis, a lung disease caused by the bacteria Chlamydia psittaci.

4) Viral Pneumonia

    -Influenza. Pneumonia is a major complication of the flu and can be very serious(usually staphylococcal)
    -Respiratory syncytial virus usually infects infants and causes bronchiolitis. It can also cause pneumonitis.
    -Parainfluenza virus, Adenoviruses, Herpes viruses, Avian influenza(Type A influenza subtype H5N1 in birds/Swine flu) are other relatively uncommon causes of pneumonia.

5) Anaerobic Bacteria

This usually results from aspiration. This may happen in children and adults with chronic neuromuscular disorders, after a head injury or general anesthesia, or under the effects of drugs or alcohol.

6) Opportunistic infections

    -Pneumocystis carinii, renamed Pneumocystis jiroveci in 2002, is a parasite. It was originally thought to be protozoa but is now classified as a fungus.
    -Mycobacterium tuberculosis, avium etc.
    -Cytomegalovirus

7) Chemical pneumonia:

    Exposure to chemicals can also cause inflammation and pneumonia.