Sialic acid
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 bacteria's ability to invade through the blood-brain barrier. The capsule can be variably O-acetylated.