Bacterial Meningitis: Organisms by Age, CSF Analysis & Treatment

Creator: Progress Note
Published: 2026-07-22
Keywords: bacterial meningitis treatment organisms, meningitis CSF findings, meningitis organisms by age, ceftriaxone vancomycin meningitis, meningitis lumbar puncture, meningococcal meningitis

The 60-second version

Classic triad: fever + neck stiffness + AMS — only 44% of cases; do not wait for all 3 to treat
Organisms by age: neonates (GBS, E. coli, Listeria); children/adults (N. meningitidis, S. pneumoniae); elderly/immunocompromised (add Listeria)
CSF: bacterial = PMN pleocytosis, low glucose, high protein; viral = lymphocytes, normal glucose
Empiric: ceftriaxone + vancomycin + dexamethasone (BEFORE or with antibiotics); add ampicillin for Listeria coverage
CT before LP only if: papilledema, focal deficits, AMS, new seizure, immunocompromised

Organisms by Age Group

Age determines the most likely pathogen — a critical fact for empiric coverage. **Neonates (<1 month)**: Group B Streptococcus (#1), E. coli (K1 capsule), Listeria monocytogenes. Treatment: ampicillin + gentamicin (or cefotaxime). **Infants 1–3 months**: GBS, E. coli, S. pneumoniae, N. meningitidis, Listeria — 'transition zone.' **Children/adults**: N. meningitidis (meningococcus — hemorrhagic/petechial rash, Waterhouse-Friderichsen syndrome with adrenal hemorrhage), S. pneumoniae (most common bacterial meningitis in adults). **Elderly/immunocompromised**: S. pneumoniae, Listeria (add ampicillin!), gram-negatives. **College students/crowded settings**: N. meningitidis — vaccinate with MCV4. **Asplenic patients**: encapsulated organisms (S. pneumoniae, N. meningitidis, H. influenzae). **HIV/immunocompromised**: Cryptococcus neoformans (fungal — India ink, cryptococcal antigen), TB meningitis (basilar meningitis, cranial nerve palsies, low glucose).

Diagnosis: CSF Analysis

Lumbar puncture is the definitive test. **Normal CSF**: opening pressure <20 cm H2O, WBC 0–5/μL (lymphocytes), glucose 45–80 mg/dL (>60% of serum), protein 15–45 mg/dL, clear appearance. **Bacterial meningitis**: opening pressure elevated (>30 cm H2O), WBC >1000/μL (PMN predominance, 'cloudy'), glucose <45 or <40% serum glucose (bacteria consume glucose), protein >100 mg/dL, Gram stain positive in 60–90%. **Viral (aseptic) meningitis**: WBC 10–300 (lymphocyte predominance), glucose normal, protein mildly elevated. **TB meningitis**: lymphocytic pleocytosis, very low glucose, very high protein, high ADA (adenosine deaminase). **Cryptococcal meningitis**: lymphocytes, low glucose, high protein, India ink positive (50%), cryptococcal antigen positive (sensitivity >95%). When to CT before LP: papilledema, focal neurological deficits, altered consciousness, new-onset seizure, immunocompromised. CT cannot rule out herniation — a normal CT does not guarantee LP safety.

Empiric Treatment and Corticosteroids

**Do not delay antibiotics for LP or CT if meningitis is suspected** — antibiotics first, then imaging/LP. **Empiric regimen for adults**: Ceftriaxone 2g IV q12h (covers S. pneumoniae and N. meningitidis) + Vancomycin 15–20 mg/kg IV q8–12h (covers PCN-resistant S. pneumoniae) + Dexamethasone 0.15 mg/kg IV q6h × 4 days (start BEFORE or with first antibiotic dose). **Add ampicillin** for: neonates, elderly (>50), immunocompromised, pregnancy — covers Listeria (cephalosporins don't cover Listeria). **Corticosteroids** reduce mortality and neurological sequelae (hearing loss, focal deficits) in pneumococcal and Hib meningitis — most benefit shown in S. pneumoniae meningitis. Dexamethasone should be given before or with first dose of antibiotics; less benefit if given after. **Cryptococcal meningitis treatment**: induction with liposomal amphotericin B + flucytosine × 2 weeks, then fluconazole consolidation/maintenance. **Chemoprophylaxis**: rifampin or ciprofloxacin for close contacts of N. meningitidis (to eliminate nasopharyngeal carriage).

Test yourself

Quick check — 3 questions

1. A 22-year-old college student presents with fever, severe headache, neck stiffness, and a petechial/purpuric rash spreading rapidly. What is the most likely organism and immediate next step?

2. LP results: WBC 2000/μL (95% PMNs), glucose 28 mg/dL (serum glucose 90), protein 180 mg/dL. Gram stain shows gram-positive diplococci. What is the appropriate empiric regimen while cultures are pending?

3. A 68-year-old with suspected bacterial meningitis has a focal cranial nerve VI palsy. What is the appropriate sequence of actions?

Frequently asked questions

Why is ampicillin added to empiric meningitis coverage for elderly patients?

Listeria monocytogenes causes meningitis predominantly in neonates, elderly (>50 years), pregnant women, and immunocompromised patients. Listeria is intrinsically resistant to all cephalosporins (ceftriaxone will not cover it). Ampicillin is the drug of choice for Listeria. The standard empiric regimen for these groups is ceftriaxone + vancomycin + ampicillin. TMP-SMX is an alternative if PCN-allergic.

What is Waterhouse-Friderichsen syndrome?

Waterhouse-Friderichsen syndrome is bilateral adrenal hemorrhage caused by meningococcal septicemia (N. meningitidis bacteremia). The meningococcus triggers DIC and endothelial damage, causing hemorrhage into the adrenal glands, leading to acute adrenal insufficiency. Clinically: fulminant bacterial sepsis + petechial/purpuric rash + bilateral adrenal hemorrhage on CT + hypotension unresponsive to vasopressors. Treat with antibiotics + stress-dose corticosteroids.

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