Acute Bacterial Meningitis
A 51-year-old woman is admitted with two days of severe headache, now with neck pain, fever and increasing confusion. She is drowsy with neck stiffness and a positive Kernig sign, and there is no rash. Cerebrospinal fluid is turbid with 200 cells/µL that are 90% neutrophils, protein 0.9 g/L, and glucose 1.6 mmol/L against a serum glucose of 6.0 mmol/L. Which of the following is the most likely causative organism?
- A.Escherichia coli
- B.Listeria monocytogenes
- C.Streptococcus pneumoniaeCorrect
- D.Streptococcus pyogenes
Explanation
The cerebrospinal fluid profile of turbid fluid, neutrophil predominant pleocytosis, elevated protein, and a cerebrospinal fluid to serum glucose ratio well below 0.4 is diagnostic of bacterial meningitis. Streptococcus pneumoniae is the single commonest cause of community-acquired bacterial meningitis in immunocompetent adults beyond adolescence and is therefore the correct answer in a previously healthy 51-year-old with no rash. Neisseria meningitidis is the main alternative and should be suspected with a petechial or purpuric rash, which is absent here. Listeria monocytogenes becomes an important pathogen only above 50 years of age with additional risk factors such as immunosuppression, pregnancy, diabetes, alcohol use, or malignancy, and it characteristically produces a lymphocytic or mixed picture, sometimes with rhombencephalitis; empirical ampicillin is nevertheless added once a patient reaches this age band. Escherichia coli meningitis occurs in neonates, after neurosurgery, or in the profoundly immunocompromised, not in a healthy adult from the community. Streptococcus pyogenes is a rare cause and typically follows contiguous spread from otitis media, sinusitis, or a skull base defect. Empirical treatment must not wait for the lumbar puncture and consists of intravenous vancomycin plus ceftriaxone, with ampicillin added above 50 years, and dexamethasone given before or with the first antibiotic dose because it halves the rate of hearing loss and death in pneumococcal disease. Computed tomography before lumbar puncture is required only if there is a focal deficit, papilloedema, new seizures, severe immunocompromise, or a markedly depressed conscious level.
Why each option
- A.
- Escherichia coli causes neonatal, post-neurosurgical, or immunocompromised meningitis, not community-acquired disease in a healthy adult.
- B.
- Listeria is a concern above 50 years with immunosuppression or pregnancy and usually gives a lymphocytic or mixed pleocytosis.
- C.
- Correct. Pneumococcus is the leading cause of community-acquired bacterial meningitis in immunocompetent adults and matches this purulent cerebrospinal fluid profile.
- D.
- Streptococcus pyogenes meningitis is rare and generally follows contiguous spread from otitis, sinusitis, or a dural defect.
Reference: IDSA Practice Guidelines for the Management of Bacterial Meningitis, 2004; UpToDate 2025, Acute bacterial meningitis in adults
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