Q.22 Gram staining is based on the presence of ______ in the cell wall.
- Phospholipid
- Peptidoglycan
- Lipid A
- Glycolipid
Gram staining differentiates bacteria based on peptidoglycan thickness in the cell wall—Gram-positive retain crystal violet due to thick layers, while Gram-negative lose it.
Question Analysis
Gram staining classifies bacteria by cell wall properties using crystal violet, iodine, decolorizer (ethanol/acetone), and safranin counterstain.
Option Breakdown
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Phospholipid: Lipid bilayer component of cell membranes; not primary Gram determinant, as both Gram types have them.
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Peptidoglycan: Polysaccharide-peptide polymer forming cell wall mesh; thick (50-90% wall) in Gram-positive (traps dye), thin (10%) in Gram-negative (leaks dye).
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Lipid A: Endotoxin in Gram-negative outer membrane lipopolysaccharides; aids decolorization but not the staining basis.
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Glycolipid: Minor membrane sugar-lipids; irrelevant to Gram mechanism.
Answer: Peptidoglycan—core structural difference exploited by staining.
Introduction to Gram Staining
Gram staining presence cell wall peptidoglycan determines bacterial classification. Thick peptidoglycan retains purple dye in Gram-positive; thin layers reveal pink in Gram-negative.
Peptidoglycan’s Central Role
Ethanol decolorizer shrinks thick peptidoglycan, trapping crystal violet-iodine complexes. Gram-negative thin layers dissolve, losing dye.
Option Cell Wall Role Basis for Gram Staining? Phospholipid Membrane fluidity No Peptidoglycan Structural mesh Yes Lipid A Gram-neg endotoxin No Glycolipid Minor lipid variation No Clinical Importance
Identifies pathogens for antibiotic choice (e.g., penicillin targets peptidoglycan); foundational in microbiology diagnostics.
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