What is Listeria monocytogenes?

What is Listeria monocytogenes?

Listeria monocytogenes is a Gram-positive, facultative intracellular bacterial pathogen transmitted primarily through contaminated food that causes severe invasive infections including meningitis, septicemia, and pregnancy-associated complications with considerable mortality, particularly in immunocompromised patients, pregnant women, and the elderly. 1, 2, 3

Microbiological Characteristics

• L. monocytogenes is a short, anaerobic, non-spore-forming Gram-positive bacillus that produces a narrow zone of hemolysis on blood agar 3
• The organism is a facultative intracellular pathogen with complex pathogenesis, capable of invading and proliferating within macrophages and most tissue cells 3, 4
• It possesses unique virulence factors including internalin, listeriolysin O, phospholipases, and ActA protein that enable intracellular survival and cell-to-cell spread through actin polymerization 4

Clinical Syndromes

L. monocytogenes produces several distinct clinical presentations: 1, 2

• Meningitis and meningoencephalitis with fever, headache, altered mental status, and neck stiffness (though elderly patients may present with altered consciousness without fever or neck stiffness) 2
• Bacteremia/sepsis with systemic signs of infection 2
• Pregnancy-associated infections including stillbirth, miscarriage, and neonatal sepsis 1, 5
• Listeriosis of the newborn 1
• Localized infections in various sites 1
• Gastroenteritis in the absence of invasive disease 6

High-Risk Populations

Immunosuppressive therapy confers particularly high risk for L. monocytogenes infection, with anti-TNF agents appearing to carry greater risk than other immunosuppressants. 1

• Pregnant women have 10-17 times higher risk of invasive listeriosis, accounting for 17-33% of all invasive cases 2, 7
• Immunocompromised patients including those with HIV, cancer, organ transplant recipients, and patients on prolonged corticosteroids or anti-TNF therapy 1, 2
• Elderly patients 2, 3
• Patients with chronic liver disease, cirrhosis, or hemochromatosis 2
• L. monocytogenes infections after infliximab treatment frequently occur after three or fewer infusions, suggesting possible reactivation of latent infection 1

Transmission and Prevention

• The main route of transmission is consumption of contaminated food, making this a foodborne pathogen 3, 6
• Prevention requires strict food hygiene and avoidance of high-risk foods including raw eggs, unpasteurized milk, raw-milk cheese, and insufficiently cooked or raw meat 1
• High-risk individuals must avoid unpasteurized dairy products and raw or undercooked meats 7

Pathogenesis

• The bacteria cross the intestinal barrier at Peyer's patches to invade mesenteric lymph nodes and bloodstream 4
• L. monocytogenes has exceptional ability to cross three critical barriers: the intestinal barrier, the blood-brain barrier (causing neurolisteriosis), and the placental barrier (causing materno-fetal listeriosis) 3, 8
• The main target organ is the liver, where bacteria multiply inside hepatocytes, leading to prolonged septicemia particularly in immunocompromised hosts 4
• Elective location of infection foci in the brain stem (rhombencephalitis) determines prognosis 4

Clinical Outcomes

• Mortality is considerable, ranging from 20-30% in high-risk individuals despite appropriate antibiotic therapy 1, 6, 4
• The overall mortality remains high (25-30%) even with bactericidal antibiotic therapy, largely due to severity of meningoencephalitis 4
• Listeria is more likely to cause death than other bacteria causing food poisoning 6

Critical Clinical Pitfall

A high index of suspicion is mandatory for patients on immunosuppressive therapy presenting with signs of meningitis or neurological symptoms—comprehensive investigation including lumbar puncture must be performed immediately as early diagnosis and treatment are critical given the pathogen's high mortality. 1, 7