Question

why do we use positive and negative controls in the gram stain?
a) to exclude gram-variable bacteria
b) to verify that the gram stain was performed properly
c) to determine if an unknown sample is gram-positive or negative
d) to distinguish between gram-positive and gram-negative bacteria

Explanation:

Positive controls (known Gram - positive bacteria) and negative controls (known Gram - negative bacteria) are used in the Gram stain. If the Gram - positive control stains purple (as it should) and the Gram - negative control stains pink (as it should), it shows that each step of the Gram - staining procedure (crystal violet application, iodine mordant, alcohol decolorization, safranin counter - stain) was done correctly. If the controls do not stain as expected, there was an error in the staining process.

Answer:

B. To verify that the Gram stain was performed properly