Pneumoniae

Streptococcus pneumoniae, often referred to by the species epithet pneumoniae, is a Gram-positive, alpha-hemolytic diplococcus that is encapsulated and fastidious in culture. It is part of the human nasopharyngeal microbiota in many individuals and is transmitted by respiratory droplets. Carriage is common, particularly in young children, and can be asymptomatic or lead to disease under certain host conditions.

The organism is a leading cause of community-acquired pneumonia and is also a major cause of meningitis, acute otitis media, sinusitis, and invasive bacteremia. Disease risk is higher in young children, older adults, individuals with chronic illnesses, asplenia, or immunosuppression. Clinical presentation ranges from upper respiratory infections to lobar pneumonia and life-threatening sepsis.

Key virulence features include a polysaccharide capsule that determines serotype and resists phagocytosis, as well as pneumolysin and autolysins that contribute to tissue damage and inflammation. The capsule’s diversity underlies serotype-related differences in virulence and vaccine coverage. The organism adheres to and invades mucosal surfaces, enabling colonization and potential spread to sterile sites.

Diagnosis combines clinical assessment with laboratory tests. Cultures of sputum, blood, or cerebrospinal fluid, Gram stain, and antigen tests (such as urinary pneumococcal polysaccharide antigen) support identification; PCR methods are increasingly used. Treatment depends on disease severity and local resistance patterns; ampicillin or high-dose amoxicillin has been standard, but penicillin resistance in some strains necessitates alternative beta-lactams or macrolides, cephalosporins, and adjunctive therapies for invasive disease.

Prevention centers on vaccination. Pneumococcal conjugate vaccines (PCV13, PCV15, PCV20) target common serotypes and reduce carriage and disease, while the polysaccharide vaccine PPSV23 covers additional serotypes. Vaccination, along with antibiotic stewardship, has reduced invasive pneumococcal disease in many regions.