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The most common causes of pneumonia are generally the different species of bacteria. It is often difficult to identify the specific species of bacteria that cause pneumonia. Usually, the categorization of the species of bacteria is done by a staining procedure called the Gram Stain. In this procedure, a smear of bacteria is treated to Gram Stain and then it is examined under the microscope. The color of the bacteria after the staining procedure, determines if they are gram-negative or gram-positive bacteria. This gives a clear idea of the characteristics of the bacteria and thus the physician can judge the severity of pneumonia.
Gram-Positive Bacteria: In the Gram Stain procedure, these types of bacteria appear blue in color. The following are the common gram-positive bacteria:
- Staphylococcus aureus, another gram-positive bacterium is responsible for 10% of bacterial pneumonia cases. Nosocomial pneumonia that occurs in hospitals can be attributed to this strain of bacteria. It is not common in normal healthy individuals and usually infects susceptible individuals. It can develop, five days after viral influenza, in people with weak immune systems. Children, hospitalized patients, and drug abusers who use needles are also susceptible to this strain of bacteria.
- Gram-positive Streptococcus pneumoniae is the most common cause of pneumonia. This bacterium is also called as S. pneumoniae or pneumococcal pneumonia. In the beginning, it was assumed that 95% of community-acquired bacterial infection was caused by this species, but recent studies have revealed that the cases were far less, accounting for approximately half of all cases. Some research studies suggest that it may be only responsible for even fewer cases (10% to 30% of cases) than what was earlier reported.
- Streptococcus pyogenes or Group A Streptococcus are also responsible for causing pneumonia.
Gram-Negative Bacteria: In the Gram Stain procedure, these bacteria stain pink in color. Gram-negative bacteria commonly cause diseases in hospitalized or nursing home patients, children with cystic fibrosis, and people with chronic lung conditions.
- Moraxella catarrhalis, is normally found in the nasal and oral passages. Research reveals that this bacterium is a cause of certain pneumonias, especially in people with lung problems, like asthma or emphysema.
- Klebsiella pneumoniae, usually causes pneumonia in alcoholics and other people, who are physically debilitated.
- Haemophilus influenzae is the most common gram-negative species that cause pneumonia, generally in patients with chronic lung disease, older patients, and alcoholics.
- Neisseria meningitidis causes meningitis, which is the central nervous system infection. However, this organism has been reported in cases of pneumonia, especially in epidemics occurring in military recruits.
- Pseudomonas aeruginosa is responsible for causing pneumonia in hospitals (nosocomial pneumonia). It is commonly seen in pneumonia patients suffering from chronic or severe lung disease.
- Other gram-negative bacteria that cause pneumonia are E. coli, mostly in newborns, Proteus and Enterobacter.
Streptococcus pneumoniae also called as pneumococcus, is the most common cause of bacterial pneumonia. It causes lobar pneumonia by attacking an entire lobe or portion of a lobe of the lung. In case of double pneumonia, this bacterium attacks both the lungs. Pneumococcal lobar pneumonia usually occurs in winter after a severe upper respiratory viral infection. The most common symptoms are chills followed by high fever i.e about 40°C (104°F), chest congestion and pain in the chest while breathing and cough with blood-streaked sputum.
Other bacteria that are known to cause pneumonia are Klebsiella pneumoniae, Haemophilus influenzae, Legionella pneumophilia (the bacterium that causes Legionnaires' disease), and various species of staphylococci and streptococci bacteria. These organisms primarily cause bronchopneumonia and the onset of symptoms is usually slower than with lobar pneumonia.
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