What is the empiric antibiotic therapy for suspected Listeria (Listeria monocytogenes) infections?

Empiric Antibiotic Therapy for Suspected Listeria Infections

For suspected Listeria infections, immediately initiate high-dose intravenous ampicillin (or amoxicillin) 2 g every 4 hours PLUS gentamicin for synergistic bactericidal effect, particularly in high-risk patients including those >50 years, pregnant women, immunocompromised individuals, or those with specific risk factors. 1, 2

Age-Based and Risk-Stratified Approach

Neonates (<1 month old)

• Ampicillin/amoxicillin 50 mg/kg every 6-8 hours PLUS cefotaxime 50 mg/kg every 6-8 hours (age-dependent dosing) 1
• Alternative: Ampicillin PLUS an aminoglycoside (gentamicin 2.5 mg/kg every 8-12 hours depending on age) 1

Children and Adults Age 18-50 Years WITHOUT Risk Factors

• Standard regimen: Ceftriaxone 2 g every 12 hours (or 4 g every 24 hours) OR cefotaxime 2 g every 4-6 hours PLUS vancomycin (to achieve trough 15-20 μg/mL) 1
• ADD ampicillin 2 g every 4 hours if Listeria coverage desired, though only 1.5% of this population develops Listeria without risk factors 1

Adults >50 Years OR Any Adult with Listeria Risk Factors

• Mandatory triple therapy: Ceftriaxone 2 g every 12 hours PLUS vancomycin (trough 15-20 μg/mL) PLUS ampicillin 2 g every 4 hours 1
• Risk factors requiring Listeria coverage include: diabetes mellitus, immunosuppressive drugs, cancer, other immunocompromise, alcoholism, chronic liver disease 1

Specific Clinical Scenarios

Meningitis/CNS Infections

• Ampicillin 2 g IV every 4 hours PLUS gentamicin is the treatment of choice 1, 3, 4, 5
• The combination provides synergistic bactericidal activity that ampicillin alone lacks 5, 6
• Duration: 2-3 weeks for meningitis, with most UK patients treated for approximately 20 days 5
• High doses (>6 g/day) are essential due to poor CSF penetration 5

Bacteremia/Septicemia

• Ampicillin 2 g IV every 4 hours PLUS gentamicin 1, 3, 7
• Duration: 1-2 weeks for uncomplicated bacteremia 5
• Alternative: Penicillin G at equivalent high doses 7, 4

Invasive Gastroenteritis

• High-dose IV ampicillin or penicillin G for 2 weeks in high-risk patients with fever, diarrhea, and systemic symptoms 2, 7
• Most cases of acute watery diarrhea in immunocompetent adults do not require empiric antimicrobials, but high-risk patients (pregnant, immunocompromised, elderly) require immediate treatment 2

Pregnancy

• Ampicillin or amoxicillin is the preferred agent 1, 2
• Alternative: Trimethoprim-sulfamethoxazole (though avoid in first trimester and near term) 1, 2
• Avoid fluoroquinolones during pregnancy 2
• Pregnant women have 10-17 times higher risk of invasive listeriosis 8

Alternative Regimens for Penicillin Allergy

First Alternative

• Trimethoprim-sulfamethoxazole is the best alternative for penicillin-allergic patients 1, 4, 5
• Excellent CSF penetration and bactericidal activity 5

Other Alternatives (Less Preferred)

• Vancomycin for bacteremia only (inadequate CSF penetration for meningitis) 4, 5
• Erythromycin may be used in pregnancy 5
• Meropenem as alternative 7
• Cephalosporins are NOT active against Listeria and should never be used as monotherapy 4

Critical Timing Considerations

• Antibiotic therapy must be initiated within 1 hour of clinical suspicion 1
• Do not delay antibiotics for lumbar puncture or imaging - start empiric treatment immediately if LP is delayed 1
• Perform cranial imaging before LP only if: focal deficits (excluding cranial nerve palsies), new seizures, severely altered mental status (GCS <10), or severe immunocompromise 1

Common Pitfalls to Avoid

• Never use cephalosporin monotherapy - cephalosporins lack activity against Listeria despite being standard for other bacterial meningitis 4
• Don't underdose ampicillin - requires >6 g/day due to poor CNS penetration and weak bactericidal activity as monotherapy 5, 6
• Don't forget gentamicin - the combination is synergistic and significantly more effective than ampicillin alone 1, 5, 6
• Don't overlook risk factors - age >50 alone mandates Listeria coverage regardless of other factors 1
• Don't stop immunosuppression without consideration - temporarily withhold in immunocompromised patients until infection resolves 2

Monitoring and Duration

• Monitor vancomycin trough levels (target 15-20 μg/mL) when used 1
• Monitor aminoglycoside levels when appropriate 5
• Adjust doses for altered organ function 5
• Endocarditis requires 6-8 weeks of therapy 5