Pneumococci

Pneumococci, or Streptococcus pneumoniae, are Gram-positive, facultative anaerobic cocci that occur as lancet-shaped diplococci. They are alpha-hemolytic on blood agar and are optochin sensitive and bile soluble; they are catalase‑negative and commonly found as part of the human nasopharyngeal flora.

Most people carry pneumococci in the upper respiratory tract for varying periods, with higher carriage rates in children. Transmission occurs via respiratory droplets and close contact, and carriage can seed both mucosal and invasive infections.

The capsule polysaccharide is the principal virulence factor, with more than 90 capsular serotypes identified. Encapsulation helps pneumococci resist phagocytosis, contributing to invasive disease. Non-encapsulated strains also exist and are more often associated with mucosal infections. Other virulence factors include pneumolysin, autolysins, and IgA protease, which facilitate tissue invasion and immune evasion.

Clinical disease caused by pneumococci ranges from non-invasive to invasive. Non-invasive infections include otitis media, sinusitis, and bronchitis, while invasive pneumococcal disease (IPD) encompasses pneumonia with bacteremia and meningitis, and occurs across all ages but with higher risk in young children, older adults, and people with certain medical conditions.

Diagnosis relies on culture of blood, cerebrospinal fluid, pleural fluid, or sputum, supported by Gram stain and serotyping methods such as Quellung reaction or latex agglutination; rapid urinary antigen tests are available for some serotypes. Vaccination with pneumococcal conjugate vaccines (such as PCV13, PCV15, PCV20) in children and adults, and polysaccharide vaccines (PPV23) for adults and at-risk groups, has substantially reduced IPD and overall pneumococcal disease in many settings. Antibiotic therapy is guided by susceptibility testing due to regional resistance patterns.