General description

Bacterial meningitis results from infection of the central nervous system by bacteria. Although there are many causative organisms, they are characterized by age and underlying disease as follows

• Neonates to 3-month-old infants: group B Streptococcus, Escherichia coli, Staphylococcus aureus, Listeria monocytogenes
• Infants to young children from 3 months of age: Haemophilus influenzae (mostly Hib), Streptococcus pneumoniae, and Staphylococcus aureus
• Older children to adolescents: Streptococcus pneumoniae, Haemophilus influenzae, meningococci
• Adults: Streptococcus pneumoniae, meningococci
• Elderly (50 years and older): Streptococcus pneumoniae, Gram-negative rods, Listeria monocytogenes

In immunocompromised conditions, Streptococcus pneumoniae, Gram-negative rods such as Pseudomonas aeruginosa, Listeria monocytogenes, and Staphylococcus aureus (MRSA) are seen, while Staphylococcus aureus and Staphylococcus epidermidis are more common after ventricular shunting.

In most cases, the route of infection is droplet infection, with the causative organisms entering via the upper respiratory tract or respiratory tract infection foci and reaching the meninges hematogenously. In the case of group B Streptococcus infections in newborns, infection of the birth canal is also considered. Listeria monocytogenes may enter the intestinal tract, and Staphylococcus aureus and Staphylococcus epidermidis on mucous membranes and skin may reach the meninges hematogenously via catheters.