About 25,000 cases of bacterial meningitis occur annually in the United States. Of these, about 70% occur in children younger than 5 years old.10

The relative frequency with which each bacterial species causes meningitis is age-related:

Organism	Neonates	Children	Adults
Haemophilus influenzae	+	++++	+
Neisseria meningitidis	+	+++	+++
Streptococcus pneumoniae	+	++	++++
Listeria species	++	+	+
Group B Streptococci	++++	+	+
Gram-negative bacilli (E. coli)	++++	+	++
Staphylococci	+	+	++
+ = infrequent; ++ = occasional; +++ = common; ++++ = most common

In a large review (n = 493) of adult bacterial meningitis cases managed at Massachusetts General Hospital from 1962 to 1988, 40% of cases were nosocomial, of which 33% were caused by gram-negative bacilli.11 In the 60% of cases that were community-acquired, the most common pathogens were:

• Streptococcus pneumoniae (37%)
• Neisseria meningitidis (13%)
• Listeria monocytogenes (10%)

Table adapted from KL Roos, AR Tunkel, WM Scheld. Acute bacterial meningitis in children and adults. In WM Scheld, RJ Whitley, DT Durack (eds), Infections of the Central Nervous System. Philadelphia: Lippincott–Raven, 1997;336–401.
Adapted from: Goldstein MA and Harden CL. Infectious states. In: Ettinger AB and Devinsky O, eds. Managing epilepsy and co-existing disorders. Boston: Butterworth-Heinemann; 2002;83-133.
With permission from Elsevier (www.elsevier.com).