How would you treat a 78-year-old male with altered mental status and fever, whose lumbar puncture (LP) shows cerebrospinal fluid (CSF) with gram-positive bacilli, and a positive catalase test, suggestive of Listeria monocytogenes meningitis?

Treatment of Listeria Monocytogenes Meningitis in an Elderly Patient

The correct treatment for a 78-year-old male with altered mental status, fever, and CSF showing gram-positive bacilli with positive catalase test (suggestive of Listeria monocytogenes) is ceftriaxone + vancomycin + ampicillin (option C).

Rationale for Treatment Selection

Pathogen Identification

• The clinical presentation of altered mental status and fever, combined with gram-positive bacilli in CSF and positive catalase test, strongly suggests Listeria monocytogenes meningitis 1.
• Listeria is the second most common pathogen after S. pneumoniae in elderly patients, found in approximately 20% of patients with a history of cancer and 40% of patients using immunosuppressive medication 1.

Antibiotic Selection Algorithm

1. Ampicillin requirement:

   • Ampicillin is essential as it is the drug of choice for Listeria monocytogenes meningitis 2, 3.
   • Listeria is intrinsically resistant to third-generation cephalosporins (like ceftriaxone), making ampicillin necessary in the regimen 1, 3.

2. Broad coverage needed:

   • Until final culture results confirm Listeria, empiric coverage for other common causes of bacterial meningitis is necessary:
     • Ceftriaxone covers gram-negative organisms and Streptococcus pneumoniae
     • Vancomycin covers potential resistant pneumococci
     • Ampicillin specifically targets Listeria monocytogenes

3. Age-specific considerations:

   • The ESCMID guideline specifically recommends that patients >50 years should receive cefotaxime/ceftriaxone plus vancomycin/rifampicin plus ampicillin to cover Listeria 1.

Treatment Details

Optimal Antibiotic Regimen

• Ampicillin: 2g IV every 4 hours 1
• Ceftriaxone: 2g IV every 12 hours or 4g every 24 hours 1
• Vancomycin: 10-20 mg/kg IV every 8-12 hours to achieve serum trough concentrations of 15-20 mg/mL 1

Adjunctive Therapy

• Dexamethasone should be started together with antibiotic treatment in all cases of suspected bacterial meningitis 1.
• However, the benefit of corticosteroids specifically for Listeria meningitis is less established than for pneumococcal meningitis.

Important Clinical Considerations

Why Other Options Are Incorrect

• Option A (ceftriaxone + vancomycin + steroid): Inadequate as it lacks ampicillin, which is essential for treating Listeria (Listeria is resistant to cephalosporins) 1, 3.
• Option B (ceftriaxone + ampicillin + vancomycin): Same components as option C but listed in different order.
• Option D (ampicillin alone): Insufficient as monotherapy for empiric treatment before confirmation of the pathogen; combination therapy with an aminoglycoside is typically recommended for Listeria 4, 3.

Special Considerations

• The mortality rate for Listeria meningitis is high (approximately 30%), reflecting both host factors and often delayed diagnosis 5.
• Ampicillin alone exerts only weak bactericidal activity against Listeria, which is why combination therapy is often preferred 4.
• Diagnostic yield for Listeria monocytogenes in CSF is lower (25-35%) compared to other bacterial causes of meningitis 6, making empiric coverage crucial.

Duration of Therapy

• Treatment should be continued for at least 2-3 weeks for Listeria meningitis 4.
• Follow-up lumbar punctures may be necessary in cases where clinical improvement is not evident 7.

In summary, the treatment of choice for this elderly patient with suspected Listeria monocytogenes meningitis is ceftriaxone + vancomycin + ampicillin, with the addition of dexamethasone as adjunctive therapy.