Listeria monocytogenes

It is typically associated with meningitis and gastroenteritis in older age group, pregnancy, infants and in immunocompromised people. It grows well in refrigerated food, milk, (think ice cream and unpasteurized cheese for USMLE vignettes :), vegetables and raw meat.

Morphology

It is a Gram positive coccobacillus, with characteristic tumbling motility at 25 degree celsius. They are catalase positive.

Classification

Not relevant.

Human Pathology

Humans commonly get infected by the ingestion of contaminated food or mother to fetal transmission across the placenta or during childbirth. Bacterial surface proteins called Internalins A and B adhere to E cadherins and Met receptor tyrosine kinases which is followed by internalization of the bacteria. With the help of toxin Listeriolysin O (LLO), listeria escape from phagosomes into the cytosol. It grows intracellularly in the cytoplasm. It causes polymerisation of the host actin filaments and uses that force to move both within the cell and in between cells (actin rockets).

It causes meningitis in infants (hence the rationale of adding ampicillin to the antibiotic regimen of neonatal meningitis), abortions, premature labor and sepsis in pregnancy, gastroenteritis with watery diarrhea, fever, headache, joint and muscle pain (flu like symptoms). It causes meningitis and sepsis in immunocompromised individuals.

Laboratory diagnosis of Listeriosis

Diagnosis is done by clinical symptoms, Gram stain and culture. Gram stain shows Gram positive coccobacilli resembling diphtheroids (small, club shaped morphology). Culture on blood agar shows a zone of beta hemolysis. Tumbling motility at 25 degree celsius is characteristic.