In bacteriology, peptidoglycan (also called murein) is a hallmark structural polymer. In which bacterial component is peptidoglycan uniquely found (that is, not in eukaryotes or archaea)?

Introduction / Context:
Peptidoglycan is the mesh-like exoskeleton that gives bacteria their shape and resists osmotic pressure. Recognizing where peptidoglycan resides is fundamental for understanding Gram staining, antibiotic action (for example, beta-lactams), and differences between Gram-positive and Gram-negative organisms.

Given Data / Assumptions:

• The question asks for the specific bacterial structure that uniquely contains peptidoglycan.
• Consider common bacterial surface components: cell wall, cell membrane (plasma membrane), glycocalyx (capsule/slime), and spore layers.
• Focus on what is universal across most bacteria and unique compared with eukaryotes and archaea.

Concept / Approach:
The bacterial cell wall contains peptidoglycan (also called murein/mucopeptide). In Gram-positive bacteria, thick peptidoglycan forms the bulk of the wall. In Gram-negative bacteria, a thinner peptidoglycan layer sits within the periplasm between the inner and outer membranes. Eukaryotic cells lack peptidoglycan, and archaeal walls use different polymers (for example, pseudopeptidoglycan or S-layers), making peptidoglycan a bacterial signature.

Step-by-Step Solution:

Verification / Alternative check:
Gram staining outcomes (retention of crystal violet by thick peptidoglycan) and the mechanism of action of penicillins (inhibiting transpeptidation in peptidoglycan) both confirm that peptidoglycan is integral to the bacterial cell wall.

Why Other Options Are Wrong:

• Cell membrane: phospholipid bilayer; peptidoglycan is not part of the membrane.
• Glycocalyx: capsule/slime composed of polysaccharide or polypeptide, not peptidoglycan.
• Spore: cortex and coats are specialized layers; their composition and function differ and are not simply peptidoglycan wall.

Common Pitfalls:
Confusing the capsule (glycocalyx) with the cell wall; the capsule is outside the wall and chemically distinct.

Final Answer:
cell wall