A 72-year-old man is brought to the emergency department by his wife due to fever, headache, confusion, and difficulty walking for the past day. He has a history of type 2 diabetes mellitus and alcoholic cirrhosis. His HbA1c was 8.6% two months ago. He has not traveled recently and has no known sick contacts. His wife reports that he eats soft cheeses and cold deli meats regularly, including a recent batch of imported brie. He is up to date on vaccinations, including pneumococcal and influenza vaccines. Temperature is 38.8°C (101.8°F), blood pressure is 132/84 mm Hg, pulse is 92 beats/min, and respirations are 18/min. Physical examination reveals mild confusion, nuchal rigidity, and an ataxic gait. Kernig and Brudzinski signs are positive. A lumbar puncture is performed, and cerebrospinal fluid shows elevated opening pressure, 400 WBCs/mm3 (75% mononuclear), low glucose, and elevated protein. Gram stain of the CSF reveals small, gram-positive rods.
Which of the following organisms is the most likely cause of this patient’s condition?
C) Listeria monocytogenes
Listeria monocytogenes is a facultative intracellular, motile, gram-positive rod that can cause meningitis in elderly, immunocompromised individuals, and neonates. It is transmitted primarily through contaminated food, particularly unpasteurized dairy products, deli meats, and soft cheeses. Listeria is able to survive and multiply at refrigeration temperatures, which contributes to its association with refrigerated foods.
In adults, listeriosis can present as meningoencephalitis, particularly in patients with risk factors such as older age, diabetes, alcohol use disorder, and immunosuppression. This patient’s cerebrospinal fluid (CSF) findings—mononuclear pleocytosis, low glucose, high protein, and gram-positive rods—are characteristic of Listeria central nervous system (CNS) infection.
Answer choice A: Cryptococcus neoformans, is incorrect. Cryptococcus is an encapsulated yeast that causes meningitis primarily in severely immunocompromised patients (e.g., AIDS with CD4 <100 cells/mm3). CSF typically shows lymphocytic predominance, low glucose, and elevated protein, but India ink staining or cryptococcal antigen detection, not Gram stain, are diagnostic. This patient’s gram stain shows gram-positive rods, not yeast.
Answer choice B: Haemophilus influenzae type b, is incorrect. H. influenzae type b is a gram-negative coccobacillus that used to be a common cause of pediatric meningitis but is now rare due to vaccination. It primarily affects unvaccinated children. This patient is an elderly adult with recent exposure to high-risk foods and has evidence of an organism that stains as gram-positive rods, not a gram-negative coccobacillus.
Answer choice D: Neisseria meningitidis, is incorrect. Neisseria meningitidis is a gram-negative diplococcus and a leading cause of bacterial meningitis in adolescents and young adults. It often presents with petechial rash and rapid progression. The patient’s age, CSF findings, and gram stain do not support this diagnosis. N. meningitidis would appear as gram-negative diplococci on Gram stain.
Answer choice E: Streptococcus pneumoniae, is incorrect. S. pneumoniae is the most common cause of community-acquired bacterial meningitis in adults, and it appears as gram-positive lancet-shaped diplococci. This patient’s gram-positive rods, along with dietary risk factors and immune status, point toward Listeria rather than pneumococcus.
Key Learning Point
Listeria monocytogenes is a gram-positive rod that causes meningitis in elderly and immunocompromised patients. It is transmitted through contaminated foods such as unpasteurized dairy and deli meats. It should be suspected in patients with meningoencephalitis and gram-positive rods in CSF, particularly with compatible risk factors and exposures.
