Which bacteria is the main cause of neonatal meningitis in infants under 4 weeks old?

Escherichia coli is recognized as the main cause of neonatal meningitis in infants under 4 weeks old due to its presence in the gastrointestinal tract of pregnant women. When E. coli strains, particularly those that are K1 serotype, penetrate into the bloodstream during pregnancy or immediately before or after birth, they can lead to serious infections, including meningitis in neonates. This risk is heightened in premature infants or those with lower birth weights, as their immune systems are not fully developed.

While other bacteria listed can also cause meningitis, they tend to be associated with different age groups or circumstances. For instance, Streptococcus pneumoniae and Listeria monocytogenes are more commonly associated with later onset meningitis in older infants and adults, while Haemophilus influenzae infections have declined significantly in the post-vaccine era. Therefore, the particular characteristics of E. coli infection in neonates highlight its prominence as a leading cause of meningitis in this vulnerable population.