A student is performing a Gram stain of a mixed culture of both E. coli and S. aureus and he forgets to decolorize with ethanol. What should his slide look like? A. Both are colorless because the ethanol is needed for the cells to absorb the stains B. The E. coli should appear red/pink and the S aureus should appear blue/purple. C. Both the E. coliand the S. aureus should appear blue/purple. D. Both the E. coliand the S. aureus should appear red/pink.

Correct option is C.

Explanation:

Gram staining is a technique of separation of the bacteria based on the cell wall composition. Steps involved in gram staining are:

1) Primary stain application which is known as crystal voilet, it stains all cells purple/blue.

2) Mordant stain application which is known as iodine solution, is added, and form crystal iodine complex, all cells continue to stain blue/purple.

3) Decoloration step, in this step, we can differentiate gram negative or gram positive bacteria. So, decolorizing agent such as ethanol or acetone extracts blue color from thin walled peptidoglycan layer gram negative bacteria, and gram positive bacteria remain blue/purple because it contains thick walled peptidoglycan layer.

4) Counterstain application in this step safranin red dye stain the gram negative decolorized cells red/pink, and gram positive bacteria remain same blue.

So, if a student is performing a Gram stain of a mixed culture of both E. coli and S. aureus, and he forgets to decolorize with ethanol Both the bacteria should appear blue/purple color.