Which of these is exposed on the outer surface of a gram-negative bacterium?

The correct option is O-antigen of lipopolysaccharide (LPS).

To understand why this is the correct answer, let's break down the structural organization of the cell envelope of a Gram-negative bacterium:
1. Gram-Negative Cell Envelope Structure: Gram-negative bacteria have a distinct cell wall containing a thin peptidoglycan layer sandwiched between the inner (cytoplasmic) membrane and an asymmetrical outer membrane.
2. The Outer Membrane Components: The outer leaflet of the outer membrane is primarily composed of Lipopolysaccharide (LPS) molecules. LPS itself consists of three parts:
- Lipid A: Anchors the LPS molecule in the outer membrane.
- Core polysaccharide: Connects Lipid A to the O-antigen.
- O-antigen (or O-specific side chain): A long, repeating carbohydrate chain that extends outwards from the bacterial surface into the surrounding environment. Because of this extension, the O-antigen is the most exposed component on the very outer surface of the bacterium, making it highly accessible to the host immune system and bacteriophages.

Let's evaluate the incorrect options to understand why they are not exposed on the outer surface:
- Braun lipoprotein: This protein acts as a link between the outer membrane and the peptidoglycan layer. It is located on the inner face of the outer membrane, pointing inward toward the periplasm, and is not exposed on the cell's exterior.
- Polysaccharide portion of lipoteichoic acid (LTA): Lipoteichoic acid (LTA) is a major constituent of the cell wall of Gram-positive bacteria, not Gram-negative bacteria.
- Electron transport system components: In bacteria, the electron transport chain is located within the inner cytoplasmic membrane, which lies deep underneath the peptidoglycan layer and the outer membrane.