Bacterial cell membranes — identifying organisms with sterols In microbiology, only certain prokaryotes incorporate sterols (such as cholesterol) into their cytoplasmic membrane to increase rigidity and protect against osmotic stress. Among the following genera, which organism characteristically contains sterols in its cytoplasmic membrane?

Introduction / Context:
Sterols (for example, cholesterol) are membrane-stiffening lipids that are abundant in eukaryotic membranes but typically absent from the membranes of most bacteria. An important exception exists among wall-less bacteria such as Mycoplasma, which scavenge sterols from the host to stabilize their plasma membrane. This question checks your ability to recall that distinctive feature and to contrast Mycoplasma with common Gram-positive and Gram-negative genera that do not rely on sterols.

Given Data / Assumptions:

• We compare representative bacterial groups: Bacillus, Clostridium, Proteus, Pseudomonas, and Mycoplasma.
• Most eubacteria synthesize membranes rich in phosphoglycerides without sterols.
• Mycoplasma lack a peptidoglycan cell wall and depend on sterols for membrane integrity.

Concept / Approach:
Bacterial membrane composition is taxon-specific. Typical eubacteria do not produce or require sterols. Mycoplasma, however, have no cell wall; their single lipid bilayer would be too fragile without sterols. Incorporating host-derived sterols increases membrane rigidity, reduces permeability, and supports survival in variable osmotic conditions. This also explains intrinsic resistance to beta-lactam antibiotics, which target peptidoglycan synthesis that Mycoplasma lack entirely.

Step-by-Step Solution:
Identify the group known to be wall-less: Mycoplasma.Recall that Mycoplasma acquire sterols from the host and embed them in the cytoplasmic membrane.Exclude Bacillus and Clostridium (Gram-positive spore formers) and Proteus or Pseudomonas (Gram-negative rods), none of which require sterols in their cytoplasmic membranes.Select Mycoplasma as the only correct match.

Verification / Alternative check:
Laboratory culture media for Mycoplasma commonly include serum (a source of cholesterol), further evidencing the requirement for sterols. In contrast, standard bacterial media for Bacillus, Clostridium, Proteus, and Pseudomonas do not need sterol supplementation.

Why Other Options Are Wrong:

• Bacillus: possesses a rigid peptidoglycan wall; membranes lack sterols.
• Proteus: Gram-negative enteric; outer membrane has LPS but no cytoplasmic membrane sterols.
• Clostridium: anaerobic Gram-positive spore former; no sterols in the cytoplasmic membrane.
• Pseudomonas: Gram-negative nonfermenter; membranes do not contain sterols.

Common Pitfalls:
Confusing Mycoplasma (wall-less, sterol-requiring) with Mycobacterium (has a complex wall with mycolic acids but not membrane sterols). Also, do not conflate eukaryotic sterol-rich membranes with typical bacterial membranes.

Final Answer:
Mycoplasma (wall-less bacteria)