Cell envelope comparison – structure lacking in gram-positive bacteria Which structural component is characteristically absent from gram-positive bacterial cell envelopes?

Introduction / Context:
Gram-positive and gram-negative bacteria differ in their cell envelope architecture, which affects staining, antibiotic permeability, and virulence. Recognizing which layers are present or absent is foundational microbiology.

Given Data / Assumptions:

• Gram-positive bacteria stain purple due to thick peptidoglycan.
• Gram-negative bacteria possess an outer membrane containing lipopolysaccharide.
• Teichoic acids are a hallmark of many gram-positive species.

Concept / Approach:

Gram-positive envelopes include a cytoplasmic membrane and a thick peptidoglycan layer often containing teichoic and lipoteichoic acids. They lack the gram-negative outer membrane and its associated lipopolysaccharide layer.

Step-by-Step Solution:

Verification / Alternative check:

Electron micrographs and antibiotic permeability profiles (e.g., vancomycin vs. polymyxin) corroborate the absence of an outer membrane in gram-positives.

Why Other Options Are Wrong:

• Murein/peptidoglycan: abundant in gram-positives.
• Teichoic acid: typical in gram-positive walls.
• Plasma membrane: universal in bacteria.
• Periplasmic space: minimal/narrow in gram-positives but not equivalent to a true outer membrane.

Common Pitfalls:

• Equating “periplasm” with “outer membrane”; they are distinct concepts.

Final Answer:

Outer membrane