Pneumonia - Causes

Causes

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

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 whether they are Gram-negative or Gram-positive. Knowing which group the bacteria belong to helps determine the severity of the disease, and how to treat it. Different bacteria are treated with different drugs.

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

• Streptococcus (S.) pneumoniae (also called pneumococcus), the most common cause of pneumonia. This Gram-positive bacterium causes 20 - 60% of all community-acquired bacterial pneumonia (CAP) in adults. Studies also suggest it causes 13 - 38% of CAP in children.
• Staphylococcus (S.) aureus, the other major Gram-positive bacterium responsible for pneumonia, causes about 2% of CAP and 10 - 15% of hospital-acquired pneumonias. It is the organism most often associated with viral influenza. It can develop about 5 days after the start 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 in hospitalized 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, accounting for 3 - 10% of all cases. It generally occurs in patients with chronic lung disease, older people, and alcoholics.
• Klebsiella (K.) pneumoniae may be responsible for pneumonia in alcoholics and other people who are physically debilitated. It is also associated with recent use of very strong antibiotics.
• Pseudomonas (P.) aeruginosa is a major cause of hospital-acquired pneumonia (nosocomial pneumonia). It is a common cause of pneumonia in patients with chronic or severe lung disease.
• Moraxella (M.) 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.
• Other Gram-negative bacteria that cause pneumonia include E. coli, proteus (found in damaged lung tissue), enterobacter, Neisseria (N.) meningitidis, and acetinobacter.