What is the most common organism causing bacterial meningitis in infants under 1 month old?

The most common organism causing bacterial meningitis in infants under 1 month old is Streptococcus agalactiae, also known as Group B Streptococcus (GBS). Infants are particularly vulnerable to infections in the first month of life due to immature immune systems and potential exposure during delivery.

Group B Streptococcus is a leading cause of meningitis in this age group, as it can be transmitted from the mother during labor and delivery if she is colonized with the bacteria. This highlights the importance of screening pregnant women for GBS colonization and administering antibiotics prophylactically when indicated, to reduce the risk of infection in the newborn.

The other organisms, though associated with bacterial meningitis in different age groups or populations, are less common in this specific age range. For instance, Escherichia coli can also cause meningitis in neonates but ranks below GBS in prevalence in this specific age group. Streptococcus pneumoniae and Haemophilus influenzae are more commonly associated with bacterial meningitis in older infants and children, particularly after the first month of life.