Pneumonia - Causes

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Causes

Bacteria are the most common cause of pneumonia. However, pneumonia can also be caused by viruses, fungi, and other agents. It is often impossible to identify the specific culprit.

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Many bacteria are grouped into one of two large categories by the laboratory procedure used to look at them under a microscope. The procedure is known as Gram staining. Bacteria are stained with special dyes, then washed in a special solution. The color of the bacteria after washing determines if they are gram-negative or gram-positive bacteria. Knowing which group the bacteria belong to helps your doctor determine how severe the disease is, and how to treat it. Different bacteria are treated with different drugs.

Gram-Positive Bacteria. These bacteria appear blue on the stain and are the most common organisms found in pneumonia. They include:

Streptococcus pneumoniae (also called S. pneumoniae or the pneumococcus) is the most common cause of pneumonia . Thisgram-positive bacteriumcauses about 20 - 60% of all community-acquired bacterial pneumonias (CAPs) in adults. Studies also suggest it causes between 13% to 38% of CAP in children.
Staphylococcus (S.) aureus, the other major gram-positive bacterium responsible for pneumonia, causes about 2% ofCAPs and between 10 - 15% of hospital-acquired pneumonias. It is the organism most often associated with viral influenza, and can develop about five days after the onset of flu symptoms. Pneumonia from S. aureus most often occurs in people with weakened immune systems, very young children, hospitalized patients, and drug abusers who use needles. It is uncommon in healthy adults.
Streptococcus pyogenes or Group A Streptococcus.

Gram-Negative Bacteria. These bacteria stain pink. Gram-negative bacteria commonly cause infections inhospitalized or nursing home patients, children with cystic fibrosis, and people with chronic lung conditions.

Haemophilus (H.) influenzae is the second most common organism causing community acquired pneumonia and accounts for 3% to 10% of all cases (generally occurring in patients with chronic lung disease, older patients, and alcoholics).
Klebsiella pneumoniae may be responsible for pneumonia in alcoholics and in other people who are physically debilitated. It is also associated with recent use of potent antibiotics.
Pseudomonas aeruginosa is a major cause of pneumonia that occurs in the hospital (hospital-acquired pneumonia). It is a common cause of pneumonia in patients with chronic or severe lung disease.
Moraxella catarrhalis is found in everyone's nose and mouth. Experts have identified this bacterium as an uncommon cause of certain pneumonias, particularly in people with lung problems, such as asthma or emphysema.
Neisseria meningitidis is one of the most common causes of meningitis (central nervous system infection), but the organism has been reported in pneumonia, particularly in epidemics of military recruits.
Other gram-negative bacteria that cause pneumonia include E. coli, Proteus (found in damaged lung tissue), and Enterobacter.