What is the treatment for Listeria (Listeria monocytogenes) infection, a gram-positive rod?

Listeria monocytogenes: Gram-Positive Rod, Not Cocci

Listeria monocytogenes is a gram-positive rod (bacillus), not a gram-positive coccus, and the first-line treatment for invasive listeriosis is high-dose intravenous ampicillin or amoxicillin, often combined with gentamicin for synergistic effect. 1

Microbiological Characteristics

• L. monocytogenes is a short, facultative anaerobic, non-spore-forming gram-positive bacillus that produces a narrow zone of hemolysis on blood agar 2
• The organism may appear gram-variable in clinical specimens, leading to potential misidentification as diphtheroids, streptococci, or enterococci 3
• Any diphtheroid isolated from blood or CSF should raise suspicion for L. monocytogenes 3

Treatment Recommendations

First-Line Therapy

• Intravenous ampicillin or amoxicillin at high doses, frequently combined with gentamicin for invasive disease including meningitis and septicemia 1, 2, 4
• The combination provides synergistic bactericidal activity, particularly important in severe infections 1

Alternative Therapy for Penicillin Allergy

• Trimethoprim-sulfamethoxazole (TMP-SMX) is the preferred alternative for penicillin-allergic patients 5, 1
• Fluoroquinolones should be avoided, especially during pregnancy 1
• Third-generation cephalosporins are ineffective against Listeria and should never be used as monotherapy 6

Special Population Considerations

Pregnant Women

• Safe options include ampicillin, cefotaxime, ceftriaxone, or TMP-SMX 1
• Fluoroquinolones are strictly contraindicated in pregnancy 1
• Pregnant women represent 17-33% of invasive listeriosis cases, with a 13-17 fold increased risk compared to the general population 1

Immunocompromised Patients

• Patients on anti-TNF agents face particularly high risk for severe Listeria infections 5
• Immunosuppressive therapy should be temporarily withheld until active infection resolves 5
• Listeria infections after infliximab frequently occur within the first three infusions, suggesting possible reactivation 5

Diagnostic Approach

Microbiological Diagnosis

• CSF Gram stain has only 25-35% sensitivity for L. monocytogenes meningitis, the lowest among common bacterial meningitis pathogens 5
• Blood and CSF cultures remain the gold standard for diagnosis 5, 1
• CSF culture positivity decreases significantly if antibiotics are given before lumbar puncture 5

Clinical Presentations

• Primary syndromes include septicemia, meningitis, and rhombencephalitis (brainstem infection mimicking "circling disease" in animals) 4
• Neonatal infections occur as early-onset sepsis from chorioamnionitis or late-onset meningitis from birth canal colonization 4
• The organism crosses the intestinal barrier, placenta, and blood-brain barrier as a facultative intracellular pathogen 2, 7

Critical Clinical Pitfalls

• High index of suspicion required for immunosuppressed patients presenting with neurological symptoms or signs of meningitis 5
• Comprehensive investigation including lumbar puncture should be performed promptly when Listeria is suspected in high-risk individuals 1
• Mortality remains 24-30% despite appropriate antibiotic therapy, reflecting delayed diagnosis and immunocompromised host status 4, 7, 8
• The prognosis depends heavily on severity of brainstem involvement (rhombencephalitis) 7

Prevention Strategies

• Food hygiene and careful food choices are essential for high-risk populations 5, 1
• Avoid unpasteurized milk and dairy products, raw-milk cheese, and undercooked or raw meats 5, 1
• The primary transmission route is through contaminated food, with highest incidences in meat, poultry, and seafood products 8