What is the recommended empiric antibiotic regimen and dexamethasone dosing for suspected acute bacterial meningitis in adults, including coverage for Listeria in patients over 50 years or who are immunocompromised?

Empiric Treatment for Acute Bacterial Meningitis in Adults

For adults with suspected bacterial meningitis, immediately administer ceftriaxone 2g IV every 12 hours (or cefotaxime 2g IV every 4-6 hours) plus vancomycin 15-20 mg/kg IV every 8-12 hours, and add ampicillin 2g IV every 4 hours for all patients over 50 years or who are immunocompromised to cover Listeria monocytogenes. 1, 2

Critical Timing: The 1-Hour Rule

Antibiotics must be administered within 1 hour of hospital presentation and should never be delayed for lumbar puncture or CT imaging. 2 Delays in antibiotic treatment are strongly associated with increased mortality and poor neurological outcomes. 2, 3 Obtain blood cultures before antibiotics if possible, but do not let this delay treatment beyond the 1-hour window. 2

Age-Based Empiric Antibiotic Regimens

Adults Under 50 Years (Immunocompetent)

• Ceftriaxone 2g IV every 12 hours OR cefotaxime 2g IV every 4-6 hours 1, 2
• This provides coverage for Streptococcus pneumoniae and Neisseria meningitidis, the most common pathogens in this population 1, 2
• Add vancomycin 15-20 mg/kg IV every 8-12 hours (targeting trough concentrations of 15-20 μg/mL) if the patient has recent travel to areas with penicillin-resistant pneumococcus or local resistance rates are elevated 1, 2, 4

Adults 50 Years or Older OR Immunocompromised (Any Age)

Triple therapy is required: 1, 2, 5

• Ceftriaxone 2g IV every 12 hours OR cefotaxime 2g IV every 4-6 hours 1
• PLUS ampicillin 2g IV every 4 hours (essential for Listeria monocytogenes coverage) 1, 2, 5
• PLUS vancomycin 15-20 mg/kg IV every 8-12 hours 1, 2, 4, 5

Risk factors for Listeria include: diabetes mellitus, immunosuppressive drugs (including methotrexate), cancer, alcohol misuse, and other immunocompromising conditions. 1, 5

Dexamethasone as Adjunctive Therapy

Administer dexamethasone 10mg IV immediately with or ideally 15 minutes before the first antibiotic dose. 1, 6, 7 Continue dexamethasone 10mg IV every 6 hours for 4 days. 1

Key Points About Dexamethasone:

• Dexamethasone reduces mortality and morbidity in pneumococcal and H. influenzae meningitis 1, 6, 7
• Can still be started up to 4 hours after the first antibiotic dose if not given initially 1
• Consider discontinuing dexamethasone if Listeria monocytogenes is identified, as observational data from 252 neurolisteriosis patients showed dexamethasone within 24 hours was associated with increased mortality 1
• For N. meningitidis, there appears to be no harm or benefit, and the decision can be individualized 1

Pathogen-Specific De-escalation After Culture Results

Streptococcus pneumoniae

• Continue ceftriaxone 2g IV every 12 hours or cefotaxime 2g IV every 4-6 hours for 10-14 days total 2, 4, 5
• If penicillin-sensitive (MIC ≤0.06 mg/L), may switch to benzylpenicillin 2.4g IV every 4 hours 2
• For penicillin and cephalosporin-resistant strains, continue ceftriaxone plus vancomycin, and consider adding rifampicin 600mg IV/PO every 12 hours 2, 4

Neisseria meningitidis

• Continue ceftriaxone 2g IV every 12 hours or cefotaxime 2g IV every 6 hours for 5-7 days total 2

Listeria monocytogenes

• Continue ampicillin 2g IV every 4 hours for 21 days total 2, 5
• Stop dexamethasone immediately when Listeria is identified 1

Critical Pitfalls to Avoid

Never Delay Antibiotics for Imaging or LP

If CT imaging is indicated (focal neurologic deficits, new-onset seizures, GCS <10, or severely immunocompromised state), start antibiotics immediately before imaging. 2, 3 The mortality benefit of early antibiotics far outweighs any diagnostic information lost. 2, 7

Do Not Omit Ampicillin in High-Risk Patients

Listeria coverage is frequently missed in patients ≥50 years or immunocompromised. 2, 5 This is a common and potentially fatal error. Cephalosporins have NO activity against Listeria—ampicillin is absolutely required. 1, 2

Ensure Adequate Dosing

Use high doses to ensure adequate CSF penetration: ceftriaxone 2g every 12 hours (not lower doses), ampicillin 2g every 4 hours (not every 6 hours), and vancomycin dosed to achieve trough concentrations of 15-20 μg/mL. 1, 2, 4

Complete Full Treatment Duration

Do not stop antibiotics prematurely based on clinical improvement alone—complete the full pathogen-specific duration as microbiological cure does not equal clinical improvement. 2

Vancomycin Monitoring

• Obtain trough serum vancomycin concentrations just before the fourth dose (at steady state) 4
• Target trough concentrations of 15-20 mg/L for meningitis 1, 4
• Monitor renal function regularly during therapy 4