In Gram-negative bacteria, which component is exposed on the outer surface and serves as a key antigenic marker for serotyping?

Introduction / Context:
Gram-negative envelopes have an outer membrane containing lipopolysaccharide (LPS). The most external, variable sugar chain of LPS—the O-antigen—is a major serological marker and an important virulence determinant. Identifying which moiety is surface-exposed guides diagnostics and vaccine design.

Given Data / Assumptions:

• The organism is Gram-negative.
• We compare LPS O-antigen, LTA, Braun lipoprotein, and electron transport components.
• We want the structure exposed on the exterior face of the outer membrane.

Concept / Approach:
LPS spans the outer membrane: lipid A anchors, core polysaccharide connects, and the O-antigen extends outward into the environment. LTA is a Gram-positive cell wall/ membrane-associated polymer. Braun lipoprotein tethers the outer membrane to peptidoglycan but is not the distal, exposed polysaccharide. Electron transport proteins are typically in the inner membrane/periplasm, not the outermost surface.

Step-by-Step Solution:

Verification / Alternative check:
Serotyping of Enterobacteriaceae (for example, E. coli O157) uses O-antigen, demonstrating its surface exposure and immunogenicity.

Why Other Options Are Wrong:

• LTA: associated with Gram-positive envelopes.
• Braun lipoprotein: structural linker, not the outward polysaccharide chain.
• Electron transport system: localized to inner membrane; not exposed externally.

Common Pitfalls:
Confusing teichoic/lipoteichoic acids (Gram-positive) with LPS (Gram-negative).

Final Answer:
O-antigen of lipopolysaccharide (LPS)