Which of the following is a common cause of bacterial meningitis in patients over 50?

• A. Streptococcus pneumoniae
• B. Staphylococcus aureus
• C. Escherichia coli
• D. Neisseria meningitidis

Answer: (A)

Streptococcus pneumoniae is the most common cause of bacterial meningitis in adults over 50. This encapsulated gram-positive diplococcus often colonizes the oropharynx and can invade the meninges, with risk heightened by age-related immune changes and comorbidities. Other organisms listed fit different contexts: Staphylococcus aureus is typically seen in nosocomial meningitis after neurosurgery or head trauma, Escherichia coli is a classic neonatal meningitis pathogen (and can appear in hospital-acquired cases), and Neisseria meningitidis more commonly causes meningitis in children and young adults and during outbreaks. Therefore, Streptococcus pneumoniae best explains meningitis in this age group.