Final answer:
The most common bacterial cause of community-acquired pneumonia is Streptococcus pneumoniae, which also causes pneumococcal meningitis. It is part of the normal respiratory microbiota but can lead to serious infections, especially in susceptible groups.
Step-by-step explanation:
Streptococcus pneumoniae (often referred to as pneumococcus) is the most common bacterial cause of community-acquired pneumonia and other diseases such as pneumococcal meningitis, septicemia, osteomyelitis, and endocarditis.
This gram-positive bacterium is part of the human respiratory tract's normal microbiota but can become pathogenic, particularly in susceptible individuals like newborns, the elderly, or those with immunodeficiencies. Pneumonia caused by S. pneumoniae is characterized by an inflammatory response in the lungs, where the alveoli fill with exudates rich in neutrophils and red blood cells, leading to a productive cough with bloody sputum.
In addition to pneumonia, this organism can cross the blood-brain barrier, causing pneumococcal meningitis, an infection resulting in inflammation of the meninges. In contrast, Mycoplasma pneumoniae, which causes primary atypical pneumonia or "walking pneumonia," is not found as part of the normal respiratory tract microbiota and tends to cause milder symptoms.