A 12-day-old neonate presents with fever, poor feeding, and a bulging fontanelle. CSF culture grows Escherichia coli. The strain is later typed as possessing the K1 capsular antigen. The property of this antigen that best explains its association with neonatal meningitis is:

• A It resists complement-mediated lysis by binding factor H
• B It functions as an IgA protease that degrades secretory IgA in the gut
• C It is a polymer of sialic acid that mimics neural tissue antigens and is poorly immunogenic ✓
• D It mediates adherence to uroepithelial cells through Gal-Gal receptors

Explanation

The K1 antigen is a homopolymer of alpha-2,8 linked sialic acid, chemically identical to the group C polysaccharide of Neisseria meningitidis and to fetal neural cell adhesion molecules. This molecular mimicry makes it poorly immunogenic, so maternal antibody against K1 is often absent, allowing hematogenous seeding of the meninges. Complement resistance contributes to virulence generally, but mimicry and poor immunogenicity are the features specific to K1.

Reference: Jawetz, Melnick and Adelberg Medical Microbiology, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.