What is the most common cause of meningitis in adults?

The most common cause of meningitis in adults is Streptococcus pneumoniae. This organism is a significant pathogen responsible for bacterial meningitis, particularly in adults and the elderly.

Streptococcus pneumoniae is a gram-positive cocci that can cause various infections, including pneumonia, otitis media, and meningitis. The prevalence of pneumococcal meningitis is notably high in adults, especially those with predisposing factors such as underlying immunocompromised states, alcoholism, or splenic dysfunction. The organism's polysaccharide capsule contributes to its virulence by helping it evade the host's immune system.

While Neisseria meningitidis is a common cause of meningitis, especially in younger populations and during outbreaks, it is less frequent compared to Streptococcus pneumoniae in the adult population. Other organisms like Listeria monocytogenes are particularly associated with meningitis in immunocompromised patients and the elderly, and Escherichia coli is more frequently linked with neonatal meningitis than with adults. Thus, while Neisseria meningitidis can indeed cause meningitis in adults, the overall incidence and commonality lean towards Streptococcus pneumoniae as the leading cause.