Which type of bacteria has a thick peptidoglycan layer and stains purple in Gram staining?

Question

BlackTom AI · Accepted Answer

A thick peptidoglycan cell wall retains the crystal violet–iodine complex during the Gram stain, so Gram-positive bacteria appear purple.

1) Relevant knowledge points and concepts
- Gram staining uses crystal violet (primary stain), iodine (mordant), alcohol/acetone (decolorizer), and safranin (counterstain).
- Peptidoglycan is a multilayered polymer in bacterial cell walls. Gram-positive bacteria have a much thicker peptidoglycan layer (roughly 20–80 nm) than Gram-negative bacteria (about 2–3 nm) and lack an outer membrane.
- Retention of the crystal violet–iodine complex during decolorization determines a purple (Gram-positive) versus pink/red (Gram-negative) result.
- Acid-fast organisms have mycolic acids and require Ziehl–Neelsen (acid-fast) staining; Mycoplasma lack a cell wall and do not retain Gram stain.

2) Step-by-step reasoning
- Step 1: Crystal violet is applied and penetrates all cells.
- Step 2: Iodine forms a larger crystal violet–iodine complex inside the cells.
- Step 3: Alcohol/acetone decolorizer dehydrates the thick peptidoglycan in Gram-positive walls, causing it to trap the CV–I complex; in Gram-negative cells the thinner peptidoglycan and outer membrane allow the complex to be washed out.
- Step 4: Counterstain (safranin) colors decolorized Gram-negative cells pink, while Gram-positive cells remain purple because they retained the CV–I complex.

3) Intermediate “calculations”
- No numeric calculations are required; the qualitative difference in layer thickness explains retention versus loss of the CV–I complex.

4) Why the correct answer is correct
- Gram-positive bacteria have the thick peptidoglycan layer that traps the crystal violet–iodine complex and therefore stain purple.

5) Why other options are incorrect
- a. Acid-fast bacteria contain mycolic acids and stain red with acid-fast stains, not purple with Gram stain.
- b. Gram-negative bacteria have a thin peptidoglycan layer and outer membrane and decolorize, staining pink with the counterstain.
- c. Mycoplasma lack a peptidoglycan cell wall, so they do not retain Gram stain (often not classically Gram-positive).

6) Conclusion
- The correct answer is d. Gram-positive bacteria, which matches the provided answer.

Which type of bacteria has a thick peptidoglycan layer and stains purple in Gram staining?单项选择题

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