Which bacteria is the most common cause of meningitis in infants less than 1 month old?

Streptococcus agalactiae, also known as Group B Streptococcus (GBS), is indeed the most common cause of meningitis in infants who are less than 1 month old. This is due to several factors, including its prevalence in the maternal genital tract. Infants can acquire GBS during delivery, particularly if they are born to mothers who are colonized with the bacteria.

In neonates, especially those under one month, the immune system is still immature, making them particularly vulnerable to infections from organisms that are often less pathogenic in older children and adults. GBS is a leading cause of early-onset sepsis and meningitis in this age group, with the infection typically manifesting within the first week of life, often shortly after birth.

While Escherichia coli is also a notable cause of meningitis in neonates, especially in cases of prematurity or prolonged rupture of membranes, it is less common than GBS in terms of overall incidence. Neisseria meningitidis and Streptococcus pneumoniae are more relevant causes for meningitis in older children and adults and are not typically the primary concerns in infants less than one month old.