What is the most likely cause of meningitis in a 51-year-old medically free woman with typical bacterial cerebrospinal fluid (CSF) analysis?

Most Common Organism Causing Meningitis in a 51-Year-Old Immunocompetent Woman

The answer is A. Streptococcus pneumoniae – this is the most common cause of bacterial meningitis in adults over 50 years of age, accounting for approximately 72% of cases in this age group, while Listeria monocytogenes remains relatively rare even in older adults unless specific risk factors are present. 1, 2

Age-Specific Epidemiology

Pneumococcal disease predominates in patients over 50 years of age. 1 The UK Joint Specialist Societies guideline explicitly states that pneumococcal disease is more common in the over-50 age group, with pneumococcal meningitis carrying a mortality rate of up to 30%, which increases with age. 1

• At 51 years old, this patient falls into the 45-64 age group, which has the highest incidence of bacterial meningitis in adults (1.21 per 100,000). 1
• S. pneumoniae causes approximately 29% of identified bacterial meningitis cases overall in adults, but this proportion is significantly higher in older adults. 1, 3
• A 2022 systematic review confirmed that S. pneumoniae causes about 72% of bacterial meningitis cases in people older than 16 years. 2

Listeria Considerations in This Age Group

Listeria monocytogenes is explicitly described as "commoner in over 60s but remains rare." 1 This is a critical distinction for a 51-year-old patient:

• Listeria is the third most common cause of bacterial meningitis overall, causing only 5% of cases. 1
• The guideline specifies that Listeria is most common in older adults (over 60) and the immunocompromised. 1
• In specific risk groups (immunosuppressed patients, cancer patients, diabetics, alcoholics), Listeria becomes the second most common pathogen after S. pneumoniae, found in 20-40% of these high-risk cases. 1
• Since this patient is described as "medically free" (immunocompetent with no comorbidities), she lacks the key risk factors that would elevate Listeria probability. 1

Clinical Context

The patient presents with "typical bacterial CSF analysis," which strongly suggests bacterial rather than viral meningitis:

• Bacterial meningitis typically shows CSF pleocytosis with predominantly polymorphonuclear leukocytes, low glucose, low CSF-to-blood glucose ratio, and elevated protein. 1
• N. meningitidis incidence is highest in younger adults aged 16-25, making it less likely in a 51-year-old. 1
• The mortality rate for pneumococcal meningitis (up to 30%) is higher than for all-cause bacterial meningitis (approximately 20%), emphasizing the clinical importance of this diagnosis. 1

Important Clinical Pitfall

Do not assume Listeria coverage is needed based solely on age 51. 1 The guidelines clearly delineate that:

• Empirical ampicillin (for Listeria coverage) should be added in older patients (typically >60 years), immunocompromised patients, and those with specific risk factors. 2
• A 51-year-old immunocompetent patient without diabetes, alcoholism, malignancy, or immunosuppressive therapy does not meet the threshold for routine Listeria coverage. 1
• However, if the patient deteriorates or fails to respond to standard therapy, Listeria should be reconsidered and ampicillin added. 1