Diagnostic microbiology — The Gram stain differentiates bacteria based on their cell wall’s ability to do what during the decolorization (solvent) step?

Introduction / Context:
The Gram stain is a foundational differential staining technique that categorizes bacteria into Gram-positive and Gram-negative groups. This classification correlates with important biological and clinical properties, including antibiotic susceptibility and virulence mechanisms, because it reflects fundamental differences in cell wall architecture.

Given Data / Assumptions:

• Crystal violet is the primary stain; iodine is the mordant; alcohol or acetone is the decolorizer; safranin is the counterstain.
• Decolorization is the critical step that differentiates organisms.
• Outcome depends on peptidoglycan thickness and outer membrane presence.

Concept / Approach:
Gram-positive bacteria have thick, multilayered peptidoglycan that retains the crystal violet-iodine complex despite solvent extraction. Gram-negative bacteria possess a thin peptidoglycan layer and an outer membrane rich in lipids; solvent treatment extracts lipids and increases permeability, allowing the primary dye complex to escape, so they do not retain crystal violet and instead take up the counterstain safranin. Thus, the Gram reaction is based on retention of crystal violet during solvent treatment, not on affinity for safranin per se.

Step-by-Step Solution:

Verification / Alternative check:
Electron microscopy and biochemical analyses confirm thicker peptidoglycan in Gram-positives and outer membranes in Gram-negatives, matching staining behavior.

Why Other Options Are Wrong:

• Retaining safranin: all decolorized cells can take up safranin; it is not the basis of differentiation.
• Retain part of both dyes: the diagnostic criterion is retention of crystal violet, not mixed retention.
• None of above: incorrect because option describing crystal violet retention is accurate.

Common Pitfalls:
Over-decolorization or under-decolorization can yield false reactions; standardized timing and solvent control are essential.

Final Answer:
to retain the crystal violet dye during solvent treatment.