Question

The choices describe possible scenarios and outcomes from Gram staining with standard stains (e.g., crystal violet and safranin). Match the scenario with the most likely outcome and explain why this occurs.

A. A gram-negative cell is stained, but the ethanol step is skipped.	i. The cell will probably appear pink, which will not correctly indicate its cell wall structure.
B. A gram-positive cell with a severely damaged cell wall is stained.	ii. The cell will appear pink.
C. A healthy gram-negative cell is stained.	iii. The cell will appear purple/violet.
D. A healthy gram-positive cell is stained.	iv. The cell will probably appear purple/violet, which will not correctly indicate its cell wall structure.

Answer 1

A. A gram-negative cell is stained, but the ethanol step is skipped.

Ans: iv. The cell will probably appear purple/violet, which will not correctly indicate its cell wall structure.

Gram-negative cell take up gram stain, however they can be easily decolorised by ethanol and later counter stained by saffranin which gives it pink color. However, when ethanol is not used, the counter stain cannot be absorbed and hence the cell will appear purple.

B. A gram-positive cell with a severely damaged cell wall is stained.

Ans: iv. The cell will probably appear pink, which will not correctly indicate its cell wall structure.

Cell wall is necessary for gram staining the cell. If cell wall of a Gram positive bacteria is damaged, peptidoglycan layer is damaged. So if such cell is stained, it can not take up crystal violet+Gram's Iodine complex. And it is stained as Gram negative i.e. pink/red.

C. A healthy gram-negative cell is stained

Ans: ii. The cell will appear pink.

D. A healthy gram-positive cell is stained.

Ans: iii. The cell will appear purple/violet