What are common etiologies of community-acquired meningitis in adults and how do risk factors guide empiric therapy?

• A. HIV is the primary cause
• B. S. pneumoniae, Neisseria meningitidis, and Listeria in older adults or immunocompromised; ampicillin added for Listeria
• C. Only viral pathogens cause meningitis in adults
• D. Pseudomonas is the most common cause everywhere

Answer: (B)

The key idea is that the bacteria most likely to cause community-acquired meningitis in adults shifts with age and immune status, and this guides what we start empirically.

Streptococcus pneumoniae and Neisseria meningitidis are the two most common bacterial pathogens causing community-acquired meningitis in adults. However, in older adults and in people who are immunocompromised, Listeria monocytogenes becomes a significant concern. Listeria is not reliably covered by standard cephalosporins, so adding ampicillin to the empiric regimen is important for those at higher risk (typically age over ~50 or with immunosuppression). By recognizing these risk factors, clinicians tailor initial therapy to cover the likely organisms and reduce the chance of missing Listeria, which can be particularly dangerous in these groups.

In practical terms, empiric therapy for adults with community-acquired meningitis often includes coverage for pneumococcus and meningococcus, with ampicillin added when the patient is older or immunocompromised to ensure Listeria is treated. While viral meningitis is indeed common and important to consider, the question focuses on the common bacterial etiologies and how risk factors shape initial treatment decisions. Choices that overemphasize HIV as the primary cause or claim a single pathogen describes all cases, or suggest Pseudomonas as the most common cause, do not fit the typical community-acquired adult pattern as accurately.