Which bacterium is most commonly associated with bacterial meningitis in adults and the elderly?

The bacterium that is most commonly associated with bacterial meningitis in adults and the elderly is Streptococcus pneumoniae. This pathogen is a leading cause of both community-acquired and healthcare-associated meningitis in these populations. It is known for its ability to invade the central nervous system following respiratory tract infections, particularly in individuals with weakened immune systems or chronic health issues.

Streptococcus pneumoniae is encapsulated, which enhances its virulence by evading phagocytosis, and it can be rapidly lethal if not treated promptly with appropriate antibiotics. Vaccination against pneumococcal infections has significantly reduced the incidence of meningitis caused by this organism, yet the disease still occurs, particularly among populations at risk, such as the elderly or immunocompromised individuals.

Although Neisseria meningitidis is also a significant cause of meningitis, especially in younger populations or during outbreaks, it is less common in older adults compared to pneumococcus. Other bacteria listed, such as Escherichia coli and Haemophilus influenzae, contribute to meningitis as well but are not the primary pathogens associated with adult and elderly populations. E. coli often causes meningitis in neonates, while H. influenzae type b has become less of