When making a Gram stain of E. coli, the iodine step was omitted. Which of the following would be the expected result of this error?

Question

BlackTom AI · Accepted Answer

Iodine is a mordant that complexes with crystal violet (CV) to form an enlarged, insoluble CV–iodine complex that gets trapped in the peptidoglycan of Gram-positive cells and resists removal by the decolorizer. Without that mordant the CV remains loosely bound and is easily removed during the alcohol/acetone decolorization step.

Relevant knowledge points and concepts
- Gram stain steps: crystal violet (primary stain) → iodine (mordant) → alcohol/acetone (decolorizer) → safranin (counterstain).
- Iodine’s role: forms CV–iodine complexes that increase dye retention, especially in thicker peptidoglycan.
- Decolorization effect: removes loosely bound CV; without the CV–iodine complex most primary stain is washed away.
- Counterstain uptake: depends on exposed cell surface chemistry after decolorization; if primary stain and cell structures are altered, counterstain may be weak or absent.

Step-by-step reasoning
1. Apply crystal violet: E. coli cells initially take up CV and appear purple.
2. Omit iodine: no CV–iodine complexes form, so the dye remains only loosely associated with cell structures.
3. Apply decolorizer: alcohol/acetone readily removes the loosely bound CV from E. coli’s thin peptidoglycan layer, leaving the cells unstained by the primary dye.
4. Apply safranin counterstain: because the decolorization step has stripped dye and partially altered surface properties, little to no counterstain may bind; the thin-walled Gram-negative rods can therefore remain essentially unstained and appear colorless or extremely faint.

Why the correct answer is correct
The key is that omission of the mordant prevents fixation of the primary dye; the decolorizer then strips the stain away, leaving cells without a retained primary dye and often failing to take up much counterstain—so the rods can appear colorless on the slide.

Why the other options are incorrect
- “The rods would appear Gram positive”: That requires retention of CV; without iodine CV is not retained, so they will not falsely appear Gram positive.
- “The Gram stain result would be unaffected”: Omitting the mordant directly affects dye retention and therefore the result.
- “The rods would wash off of the slide, leaving it blank”: Proper heat fixation or chemical fixation prevents cells from washing off; omission of iodine does not cause cells to detach.

Conclusion
Because iodine is required to form and fix the crystal violet complex, omitting it leads to loss of dye during decolorization and leaves E. coli rods unstained; therefore the rods would be colorless, matching the provided answer.

When making a Gram stain of E. coli, the iodine step was omitted. Which of the following would be the expected result of this error?单项选择题

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