Neonatal meningitis-causing E. coli
Some strains of E. coli have a capsule, called K1, which protects the bacteria from the innate immune system and allows it to penetrate the central nervous system. The capsule contains sialic acid, which is found widely in humans and so does not set off the defenses of the body. Sialic acid also plays a role in the bacterias ability to invade through the blood-brain barrier. The capsule can be variably O-acetylated. Diagnosis
Bacterial Infection
A lumbar puncture (spinal tap) is necessary to diagnose meningitis. Cerebrospinal fluid culture is the most important study for the diagnosis of neonatal bacterial meningitis because clinical signs are non-specific and unreliable. Blood cultures may be negative in 15-55% of cases, making them unreliable as well. However, a cerebrospinal fluid to blood glucose ratio below two-thirds has a strong relationship to bacterial meningitis. A spinal tap should be done in all neonates with suspected meningitis, with suspected or proven sepsis (whole body inflammation) and should be considered in all neonates in whom sepsis is a possibility. The role of the spinal tap in neonates who are healthy appearing but have maternal risk factors for sepsis is more controversial; its diagnostic yield in these patients may be low.Early-onset is deemed when infection is within one week of birth. Late-onset is deemed after the first week. Viral Infection
Babies born from mothers with symptoms of Herpes simplex virus should be tested for viral infection. Liver tests, complete blood count, cerebrospinal fluid analysis, and a chest X-ray should all be completed to diagnose meningitis.
Herpes Simplex Virus
Herpes simplex virus is a rare cause of meningitis, occurring only 0.165 in 10,000 live births in the UK and 0.2-5 in 10,000 live births in the US Both HSV-1 and HSV-2 can cause neonatal meningitis, however, HSV-2 accounts for 70% of the cases.Herpes simplex virus is transmitted to neonates mainly during delivery (when infected maternal secretions come into contact with the baby and accounting for 85% of cases), but also occur in utero (while the fetus is still in the womb, 5% of cases) or even post-delivery, receiving the infection from the community (10% of cases). The most important factors impacting the transmission of the virus is the stage of the mothers infection (symptomatic or non-symptomatic) and the damage of any maternal membranes during birth (the longer the tissue is damaged, the higher the chance of neonatal infection). Pathogenesis
Generally, the progression of neonatal meningitis starts with bacteria colonizing the gastrointestinal tract. The bacteria then invades through the intestinal mucosa layer into the blood, causing bacteremia followed by invasion of the cerebrospinal fluid. The neonates less efficient immune system (especially the alternative complement system) lessens their defense against invading bacteria. Colonization of the mother plays an important role in transmission to the neonate, causing early-onset meningitis. Group B Streptococcus
Neonatal Streptococcus agalactiae infection is acquired in utero or during passage through the vagina. Evidence suggests that vaginal colonization  by Streptococcus agalactiae during pregnancy increases the risk of vertical transmission and early-onset disease in neonates.