Listeriosis
USMLE Step 1Difficulty: mediumLast updated:
A 27-year-old woman at 30 weeks' gestation presents with 2 days of fever, myalgias, and headache. She frequently eats sliced delicatessen meats and soft cheese that she keeps in the refrigerator. Temperature is 38.9°C (102°F). Blood cultures grow small, non–acid-fast, beta-hemolytic, gram-positive rods that show tumbling motility.
Which of the following characteristics of the causative organism best explains how contaminated refrigerated food led to her infection?
Answer and explanation
Correct answer: E. Ability to multiply at refrigerator temperatures
Fever and myalgias in a pregnant patient, with blood cultures growing beta-hemolytic gram-positive rods with tumbling motility, indicate Listeria monocytogenes. The source describes Listeria as small, non–acid-fast, nonencapsulated, nonsporulating, beta-hemolytic gram-positive bacilli with characteristic tumbling motility. Infection usually follows ingestion of contaminated dairy products, raw vegetables, delicatessen meats, or refrigerated ready-to-eat foods. The key property is the organism's ability to survive and grow at refrigerator temperatures, so lightly contaminated refrigerated food can become heavily contaminated. The source also notes that listeriosis is much more likely in pregnant patients and that treatment includes penicillin or ampicillin, often with an aminoglycoside, or trimethoprim/sulfamethoxazole.
Each option explained
- A. Formation of heat-resistant endosporesIncorrect. Listeria is nonsporulating; spore formation characterizes Bacillus and Clostridium species.
- B. A polysaccharide capsule that resists dryingIncorrect. Listeria is nonencapsulated.
- C. Production of a preformed heat-stable enterotoxinIncorrect. Preformed heat-stable enterotoxin causes rapid-onset vomiting from Staphylococcus aureus or Bacillus cereus, not invasive bacteremia.
- D. An acid-fast, mycolic acid–rich cell wallIncorrect. Listeria is non–acid-fast; mycolic acid cell walls characterize mycobacteria and Nocardia.
- E. Ability to multiply at refrigerator temperaturesCorrect. L. monocytogenes can reproduce at refrigerator temperatures, so refrigerated food becomes heavily contaminated.
Key point
Listeria grows at refrigerator temperatures; pregnant patients, neonates, older adults, and immunocompromised patients are at highest risk.
From the source
“Listeria monocytogenes can reproduce at refrigerator temperatures, so lightly contaminated refrigerated food can become heavily contaminated.”