Periplasm definition: In Gram-negative bacteria, what does the term “periplasm” specifically refer to?

Introduction / Context:
Periplasm is a compartment unique to Gram-negative bacteria (and some diderm bacteria). Its correct definition is essential to understand nutrient uptake, envelope biochemistry, and antibiotic access.

Given Data / Assumptions:

• Gram-negative envelopes include: inner (cytoplasmic) membrane, thin peptidoglycan, and an outer membrane.
• Periplasm contains enzymes, binding proteins, peptidoglycan, and transport intermediates.
• Gram-positive bacteria do not possess an outer membrane.

Concept / Approach:
The periplasm is the aqueous space between the inner and outer membranes in Gram-negative bacteria, including the thin peptidoglycan layer and periplasmic proteins. By contrast, Gram-positives lack this defined periplasmic compartment due to the absence of an outer membrane.

Step-by-Step Solution:

Verification / Alternative check:
Biochemical fractionation yields a distinct periplasmic fraction in Gram-negatives rich in periplasmic binding proteins and enzymes (e.g., beta-lactamases).

Why Other Options Are Wrong:
Gram-positive periplasm (B): Lacks an outer membrane, so no true periplasmic space. Mitochondria (C): Eukaryotic organelles, unrelated. Outside the membrane (D): Refers to external milieu/ECM, not periplasm.

Common Pitfalls:
Equating “space near the membrane” with periplasm; periplasm is a defined compartment bounded by two membranes in Gram-negative cells.

Final Answer:
the area between the inner and outer membranes of gram-negative bacteria