Listeria monocytogenes

It’s typically associated with meningitis and gastroenteritis in older age groups, pregnancy, infants, and immunocompromised people. It grows well in refrigerated foods such as milk products (think ice cream and unpasteurized cheese for USMLE vignettes), vegetables, and raw meat.

Morphology

It’s a Gram-positive coccobacillus with characteristic tumbling motility at 25°C. It’s catalase positive.

Classification

Not relevant.

Human pathology

Humans most commonly get infected by:

• Ingestion of contaminated food
• Mother-to-fetus transmission across the placenta or during childbirth

Bacterial surface proteins called Internalins A and B bind to E-cadherins and Met receptor tyrosine kinases, which triggers internalization of the bacteria. Using the toxin listeriolysin O (LLO), Listeria escapes from the phagosome into the cytosol. It then grows intracellularly in the cytoplasm.

Listeria also induces polymerization of host actin filaments and uses that force to move within a cell and spread from cell to cell (actin rockets).

Clinical disease includes:

• Meningitis in infants (this is why ampicillin is added to the antibiotic regimen for neonatal meningitis)
• Abortions, premature labor, and sepsis in pregnancy
• Gastroenteritis with watery diarrhea, fever, headache, and joint and muscle pain (flu-like symptoms)
• Meningitis and sepsis in immunocompromised individuals

Laboratory diagnosis of listeriosis

Diagnosis is based on clinical symptoms, Gram stain, and culture.

• Gram stain: Gram-positive coccobacilli resembling diphtheroids (small, club-shaped morphology)
• Culture: blood agar shows a zone of beta hemolysis
• Motility: tumbling motility at 25°C is characteristic