In most cases, infection occurs because of hematogenous seeding of the meninges and CNS. In cases of CNS or spinal anomalies (eg, myelomeningocele), there may be direct inoculation by flora on the skin or in the environment. Neonatal meningitis is often accompanied by ventriculitis, which makes resolution of infection more difficult. There is also a predilection for vasculitis, which may lead to hemorrhage, thrombosis, and infarction. Subdural effusions and brain abscess may also complicate the course.
Most organisms implicated in neonatal sepsis also cause neonatal meningitis. Some have a definite predilection for CNS infection. Group B Streptococcus (GBS) (especially type III) and the gram-negative rods (especially Escherichia coli with K1 antigen) are the most common causative agents. Galactosemia should be considered if E. coli is the causative agent in late-onset meningitis. Other causative organisms include Listeria monocytogenes (serotype IVb), other streptococci (enterococci, Streptococcus pneumoniae), other gram-negative enteric bacilli (Klebsiella, Enterobacter, and Serratia spp), and rarely Neisseria meningitides. In the very low birthweight infant, coagulase-negative staphylococci need to be considered as causative organisms in bacterial meningitis.
With CNS anomalies involving open defects or indwelling devices (eg, ventriculoperitoneal shunts), staphylococcal disease (Staphylococcus aureus and Staphylococcus epidermidis) is more common, as is disease caused by other skin flora, including streptococci and diphtheroids. Many unusual organisms, including Ureaplasma, fungi, and anaerobes, have been described in case reports of neonatal meningitis.