In adults, the most common cause of acute bacterial meningitis is?

• A. Neisseria meningitidis
• B. Streptococcus pneumoniae
• C. Haemophilus influenzae
• D. Listeria monocytogenes

Answer: (B)

In adults, the most common cause of acute bacterial meningitis is Streptococcus pneumoniae. This organism is a frequent nasopharyngeal colonizer and, when it enters the bloodstream, can seed the meninges, leading to meningitis. Its capsule helps it evade immune defenses and promotes rapid growth in the subarachnoid space, provoking the classic inflammatory CSF picture with high protein, low glucose, and neutrophilic predominance. Clinically, adults often present with sudden fever, severe headache, neck stiffness, and sometimes altered mental status, reflecting this robust inflammatory response. Among other organisms, Neisseria meningitidis is a leading cause in younger adults and can cause outbreaks, Haemophilus influenzae used to be common but is now less so due to vaccination, and Listeria monocytogenes is more relevant in the elderly, pregnant women, neonates, or immunocompromised individuals. Therefore, Streptococcus pneumoniae best explains why it’s the most frequent culprit in adult acute bacterial meningitis.