The Gram stain is useful in food microbiology and clinical settings because it differentiates bacteria based on:[Fill in the blank]

Question

BlackTom AI · Accepted Answer

Crystal violet–iodine complex retention depends on peptidoglycan thickness: bacteria with thick peptidoglycan layers (Gram-positive) retain the complex and appear purple, whereas those with thin peptidoglycan plus an outer membrane (Gram-negative) lose it and appear pink after counterstaining.

1) Relevant knowledge points and concepts
- Gram stain reagents and steps: crystal violet (primary stain), iodine (mordant), alcohol/acetone (decolorizer), safranin (counterstain).
- Cell envelope architecture: Gram-positive bacteria have thick multi-layered peptidoglycan with teichoic acids; Gram-negative bacteria have an outer membrane containing lipopolysaccharide and a much thinner peptidoglycan layer.
- Differential retention: the CV–iodine complex is trapped in thick peptidoglycan but is washed out from cells with outer membranes when decolorized.

2) Step-by-step reasoning process
- Step 1: Apply crystal violet → all cells take up the dye.
- Step 2: Add iodine → forms an insoluble CV–I complex within the cell wall.
- Step 3: Decolorize with alcohol/acetone → in Gram-positive cells, the dense peptidoglycan shrinks and traps CV–I; in Gram-negative cells, alcohol disrupts the outer membrane and the thin peptidoglycan cannot retain CV–I, so it is lost.
- Step 4: Counterstain with safranin → decolorized (Gram-negative) cells take up safranin and appear pink/red; Gram-positive remain purple.

3) Calculations
- None required.

4) Why the correct answer is correct
- The staining outcome directly reflects structural differences in bacterial cell walls (peptidoglycan thickness and presence/absence of an outer membrane), so option B matches the mechanism.

5) Why other options are incorrect
- A (aerobic vs anaerobic): oxygen preference is physiological and not revealed by Gram staining.
- C (ability to cause disease): pathogenicity depends on virulence factors, not Gram reaction.
- D (metabolic pathways): metabolic capabilities require biochemical or molecular tests, not Gram stain.

6) Conclusion
- Therefore, the Gram stain differentiates bacteria based on differences in cell wall structure, confirming answer B.

The Gram stain is useful in food microbiology and clinical settings because it differentiates bacteria based on:[Fill in the blank]单项选择题

类似问题

How are plant bacterial pathogens identified into Gram positive and negative?

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

Which of the following is TRUE concerning direct Gram stains?

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?

Which of the following is NOT a reason for inconsistent Gram stain results?

The technique of Gram-staining characterizes microbes based on what characteristic?

Within the Gram stain procedure, which particular step plays the critical role in differentiating between gram-positive and gram-negative cells?

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