Microbiology — identifying what makes Mycoplasma unique among prokaryotes Among prokaryotic microorganisms, Mycoplasma species are distinct in their cell-envelope structure. Which feature correctly explains how Mycoplasmas differ from other bacteria?

Introduction / Context:
Mycoplasmas are wall-less bacteria well known in medical and veterinary microbiology. Understanding their envelope structure explains their osmotic fragility, pleomorphism, and natural resistance to antibiotics that target peptidoglycan synthesis (for example, beta-lactams). This question tests recognition of the defining structural trait that separates Mycoplasma from typical eubacteria.

Given Data / Assumptions:

• We compare Mycoplasma cell envelopes with those of typical Gram-positive and Gram-negative bacteria.
• We focus on the presence or absence of peptidoglycan (murein) and any alternative structural components.
• Terminology: peptidoglycan is also called murein; Mycoplasma membranes often contain sterols.

Concept / Approach:
Most bacteria possess a peptidoglycan cell wall that maintains shape and protects against osmotic lysis. Mycoplasmas lack peptidoglycan entirely. Instead, they rely on a sterol-enriched plasma membrane for structural integrity, leading to pleomorphic forms and intrinsic resistance to antibiotics acting on wall synthesis. Chitin or cellulose walls belong to fungi or plants, not bacteria.

Step-by-Step Solution:
Identify that the key differentiator is the absence of peptidoglycan.Recall that Mycoplasma species have only a flexible plasma membrane and often incorporate sterols obtained from the host.Eliminate options that suggest chitin, cellulose, or a normal bacterial wall.Select the option explicitly stating the lack of a cell wall.

Verification / Alternative check:
Standard microbiology texts list Mycoplasma pneumoniae as wall-less; Gram stain is unreliable for them and beta-lactams are ineffective since there is no peptidoglycan target.

Why Other Options Are Wrong:

• A: Chitin occurs in fungi, not in bacteria.
• B: Typical bacteria have murein, but Mycoplasma lack it.
• C: Suggests a proteinaceous wall; incorrect for Mycoplasma.
• E: Cellulose walls are plant features, not bacterial.

Common Pitfalls:
Confusing Mycoplasma with Mycobacterium; the latter have peptidoglycan plus mycolic acids and are acid-fast, not wall-less.

Final Answer:
Complete absence of a cell wall, with a triple-layered plasma membrane instead