What is the most common cause of bacterial meningitis in adults?

The most common cause of bacterial meningitis in adults is Streptococcus pneumoniae. This bacterium is a leading pathogen responsible for community-acquired meningitis in adults, particularly those who are immunocompromised, aged, or have other underlying health conditions. Streptococcus pneumoniae is part of the normal flora in the nasopharynx but can invade the bloodstream and central nervous system, leading to meningitis.

This organism's ability to cause severe infections, including meningitis, is enhanced by its polysaccharide capsule, which helps it evade the host immune response. Additionally, the incidence of invasive disease caused by Streptococcus pneumoniae has not diminished significantly compared to other pathogens, even with the introduction of pneumococcal vaccines aimed primarily at children.

Other bacteria listed as potential causes of meningitis, such as Neisseria meningitidis and Haemophilus influenzae, are more prevalent in different age groups or populations and have seen incidence rates decrease with vaccination programs. Escherichia coli is also a known cause, particularly in neonates, but is less common in the adult population. Understanding the epidemiology of these pathogens is essential for appropriate diagnosis and treatment of bacterial meningitis in adults.