K1

K Antigen 

The K1 capsule is a polysialic-acid capsule, consisting predominantly of α-2,8-linked N-acetylneuraminic acid (Neu5Ac).

This is particularly interesting because the polymer resembles polysialic acid found on human cells, allowing K1-positive E. coli to appear relatively "self-like" to the immune system.

Why is K1 important?

The K1 capsule:

• inhibits complement-mediated killing
• reduces phagocytosis
• promotes survival in blood
• facilitates invasion of the CNS
• is strongly associated with neonatal E. coli meningitis
• is associated with neonatal septicemia and pyelonephritis

A landmark 1974 study found K1 in 84% of CSF isolates from neonates with E. coli meningitis

Snapshot