Question
Listeria monocytogenes.
Answer
Listeria monocytogenes is a small, Gram-positive, non-spore-forming, facultative intracellular bacillus, notable for its ability to grow at refrigeration temperatures (4°C) — an important cause of food-borne infection, particularly from unpasteurized dairy products, soft cheeses, deli meats, and contaminated raw vegetables.
Key features: exhibits characteristic tumbling motility at room temperature (25°C) but not at 37°C; shows a distinctive “umbrella-shaped” pattern of growth in semisolid motility agar; produces a narrow zone of beta-haemolysis on blood agar (can resemble Group B Streptococcus, an important differential); catalase-positive, oxidase-negative.
Pathogenesis: after ingestion, the organism crosses the intestinal epithelium, is taken up by macrophages, and uses listeriolysin O to escape the phagosome into the cytoplasm, where it multiplies and spreads directly from cell to cell (using actin-based motility, “actin comet tails”) without being exposed to extracellular antibody — an important immune-evasion strategy. It can cross both the placental and blood-brain barriers.
Clinical significance: in healthy adults, infection is often mild/subclinical (febrile gastroenteritis); high-risk groups (pregnant women, neonates, elderly, and the immunocompromised) can develop severe invasive disease — listerial meningitis/meningoencephalitis in neonates and immunocompromised adults, and in pregnancy, transplacental infection causing spontaneous abortion, stillbirth, or neonatal sepsis (granulomatosis infantiseptica).
Laboratory diagnosis: culture from blood/CSF/placental tissue on routine media (cold enrichment at 4°C improves recovery from contaminated specimens); Gram stain shows Gram-positive bacilli that can mimic diphtheroids/Corynebacterium (a recognized pitfall); confirmatory biochemical tests and motility characteristics.
Treatment: ampicillin (often combined with an aminoglycoside for synergy) — the organism is intrinsically resistant to cephalosporins, an important prescribing point since empirical cephalosporin cover for suspected neonatal/elderly meningitis will not treat Listeria.