Listeriosis
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Direct answer
Listeriosis is a food-borne infection caused by Listeria monocytogenes, a gram-positive bacillus that causes meningitis, septicaemia and miscarriage in susceptible hosts. It is most dangerous for neonates, pregnant women, older adults and immunocompromised people, and ampicillin is its key drug, since the organism is intrinsically resistant to cephalosporins.
What you must remember
- Organism: Listeria monocytogenes is a gram-positive, motile, facultative intracellular bacillus that grows even at refrigerator temperatures.
- High-risk groups: Neonates, pregnant women, older adults and immunocompromised people carry the greatest risk of serious disease.
- Clinical forms: Meningitis or rhombencephalitis, septicaemia and febrile gastroenteritis, with a flu-like illness and miscarriage risk in pregnancy.
- Neonatal disease: Transplacental or perinatal spread can cause granulomatosis infantiseptica, which is disseminated disease in the newborn.
- Drug of choice: Ampicillin, usually with gentamicin for meningitis or sepsis, is the standard combination.
- Intrinsic resistance: Cephalosporins are not active against Listeria, so they are not used as the main treatment.
- Sources: Ready-to-eat foods, soft cheeses made from unpasteurised milk and unpasteurised dairy are classic sources.
- Microbiology: Tumbling motility and narrow beta-haemolysis on blood agar help identify the organism.
Approach to suspected listerial meningitis
A 67-year-old man with diabetes presents with fever, neck stiffness and confusion. His cerebrospinal fluid shows a gram-positive rod with a mononuclear pleocytosis, and the team must decide on empirical treatment.
- Think of the host: Consider Listeria in neonates, pregnant women, older adults and immunocompromised people, since these groups are most at risk.
- Examine the CSF: Look for small gram-positive rods, seen in only a minority of cases and easily mistaken for diphtheroids, with a pleocytosis that is neutrophilic or mixed and sometimes lymphocytic.
- Culture and confirm: Grow the organism on blood agar, where it shows narrow beta-haemolysis and a positive catalase reaction.
- Start ampicillin: Give high-dose intravenous ampicillin, usually with gentamicin, rather than a cephalosporin, because Listeria is intrinsically resistant to cephalosporins.
- Adjust for allergy: Use trimethoprim-sulfamethoxazole when a penicillin allergy prevents ampicillin treatment.
- Treat pregnancy promptly: In pregnancy, treat the mother early to protect the fetus, since listeriosis can cause miscarriage, stillbirth or neonatal infection.
Where the exam tries to catch you
Most traps involve antibiotic selection, the significance of a gram-positive rod and the seriousness of pregnancy listeriosis.
- Trap: Choosing a third-generation cephalosporin for listerial meningitis. Correct: Listeria is intrinsically resistant to cephalosporins, so ampicillin is the drug of choice.
- Trap: Dismissing gram-positive rods in CSF or blood as contaminants. Correct: Listeria is a true pathogen when isolated from sterile sites, especially in susceptible groups.
- Trap: Assuming listeriosis spreads person to person. Correct: It is classically food-borne, linked to ready-to-eat foods and unpasteurised dairy.
- Trap: Treating listeriosis in pregnancy as a mild flu-like illness. Correct: It can cause miscarriage, stillbirth or neonatal infection, so it needs prompt treatment.
Frequently asked questions
Which antibiotic is the drug of choice for listeriosis?
Ampicillin is the drug of choice for listeriosis, usually given intravenously and combined with gentamicin in meningitis or sepsis. Listeria monocytogenes is intrinsically resistant to cephalosporins, so these drugs are not used as the main treatment.
Why is listeriosis important in pregnancy?
Listeriosis in pregnancy can cause a mild flu-like illness in the mother but may lead to miscarriage, stillbirth or neonatal infection. Prompt treatment of the mother is essential to protect the fetus.
Which foods are linked to Listeria infection?
Ready-to-eat foods, soft cheeses made from unpasteurised milk and deli meats are classic sources of Listeria monocytogenes. Because the organism grows at refrigerator temperatures, refrigeration alone does not prevent it.
How does Listeria appear on Gram stain and culture?
On Gram stain, Listeria monocytogenes appears as small gram-positive rods that can be mistaken for diphtheroids. On blood agar it shows narrow zones of beta-haemolysis and a positive catalase reaction.
Which CSF pattern suggests listerial meningitis?
Listerial meningitis shows a pleocytosis that is often neutrophilic but more frequently lymphocytic than in other bacterial meningitis, with raised protein and variably reduced glucose. The CSF Gram stain is positive in only a minority of cases, so the diagnosis depends on CSF or blood culture.