What are the most common organisms causing meningitis according to age group?

Common Organisms Causing Meningitis by Age Group

The causative organisms of bacterial meningitis vary systematically by age, with Group B Streptococcus and E. coli dominating in neonates, S. pneumoniae and N. meningitidis in children and adolescents, and S. pneumoniae becoming increasingly predominant in adults, with L. monocytogenes emerging as a critical pathogen in those over 60 years. 1

Neonatal Period (0-6 weeks)

Primary Pathogens:

• Group B Streptococcus (S. agalactiae) and Escherichia coli together cause approximately two-thirds of all neonatal meningitis cases 1, 2
• In European surveillance data, GBS accounts for 58% and E. coli for 21% of neonatal cases 2
• Listeria monocytogenes historically was considered important but recent surveillance shows it causes only 2% of neonatal cases 1, 2
• S. pneumoniae is only incidentally found in neonates 1

Transmission patterns differ by timing:

• Early neonatal meningitis (first week): vertical transmission through birth canal 1
• Late neonatal meningitis (weeks 2-6): nosocomial or horizontal transmission 1

Children Beyond Neonatal Age (>6 weeks to adolescence)

Current Era (Post-Vaccination):

• S. pneumoniae and N. meningitidis serogroup B are now equally common as the leading causes 1
• S. pneumoniae accounts for 22.5-41.1% of cases in children, with the highest rates in Africa 3
• N. meningitidis causes 9.1-36.2% of cases, with serogroup B now predominant after serogroup C vaccination 1

Important vaccination impact:

• H. influenzae type b has virtually disappeared as a major cause since the 1990s vaccination programs 1
• H. influenzae now occurs only incidentally in unvaccinated children or from non-type b serotypes 1
• Meningococcal serogroup C cases sharply decreased following Men C vaccine introduction 1

Adults (18-50 years)

Dominant Pathogen:

• S. pneumoniae causes the majority of adult bacterial meningitis cases 1
• Pneumococcal meningitis accounts for approximately 29% of identified cases overall but varies by region 4, 3
• N. meningitidis (mostly serogroup B) is found primarily in adolescents and young adults 1

Less common organisms:

• H. influenzae and S. aureus each cause 1-2% of adult cases 1
• H. influenzae is associated with otitis and sinusitis 1
• S. aureus is associated with endocarditis 1

Older Adults (>50-60 years)

Age-Related Pathogen Shift:

• S. pneumoniae becomes increasingly predominant, accounting for approximately 72% of cases in adults over 50 4
• Pneumococcal meningitis carries mortality rates up to 30%, which increases with age 4
• L. monocytogenes emerges as the third most common cause overall (5% of cases) but is particularly important in this age group 1, 4

Critical risk stratification for Listeria:

• In immunocompromised patients, cancer patients, diabetics, and alcoholics, Listeria becomes the second most common pathogen after S. pneumoniae, found in 20-40% of high-risk cases 4
• Listeria coverage with ampicillin should be added empirically in patients over 60 years and those with specific risk factors 4

Geographic and Epidemiologic Considerations

Regional variations exist:

• S. pneumoniae and N. meningitidis remain the predominant pathogens across all WHO regions 3
• In Africa, E. coli (17.7%) and S. pneumoniae (20.4%) are most common in neonates 3
• In Europe, N. meningitidis causes 47% of cases in children aged 1-5 years 3

Common pitfall: The epidemiology has dramatically shifted due to vaccination programs—historical data showing H. influenzae type b as a major pathogen in children is no longer applicable in vaccinated populations 1. Always consider local vaccination coverage when assessing likely pathogens.

Chronic meningitis consideration: When symptoms persist ≥4 weeks, consider spirochetes (Treponema pallidum, Borrelia species, Leptospira species) alongside M. tuberculosis and fungi 5.