Pneumococcus

Pneumococcus, or Streptococcus pneumoniae, is a Gram-positive, lancet-shaped diplococcus that is encapsulated and often alpha-hemolytic on blood agar. It is part of the normal upper respiratory tract flora in many people but can cause a range of invasive and non-invasive diseases.

The organism is a major human pathogen, responsible for community-acquired pneumonia, meningitis, and bacteremia, as well as non-invasive infections such as otitis media and sinusitis. It remains a leading cause of invasive pneumococcal disease in children and adults worldwide. Risk factors include young age, advanced age, chronic illnesses, asplenia, and immunocompromised states.

Transmission occurs through respiratory droplets, and colonization of the nasopharynx typically precedes disease. The polysaccharide capsule is the main virulence factor and defines more than 90 serotypes, which influences invasiveness and vaccine coverage. Non-encapsulated strains are generally less capable of causing invasive disease. Other virulence factors include pneumolysin, autolysins, and IgA protease.

Prevention relies on vaccination and, when needed, antibiotic therapy. Pneumococcal conjugate vaccines (PCV13, PCV15, PCV20) target multiple capsular types in children and adults, while pneumococcal polysaccharide vaccine PPSV23 covers additional serotypes. Vaccination programs have substantially reduced invasive disease and, in some settings, antibiotic resistance. Treatment typically involves beta-lactam antibiotics, but penicillin and macrolide resistance has risen in some regions, necessitating local susceptibility guidance. Diagnosis is usually by culture of blood, cerebrospinal fluid, or other sterile sites, supported by Gram stain and, when useful, antigen testing or PCR.