Question

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

Explanation:

Positive controls (known Gram - positive bacteria) and negative controls (known Gram - negative bacteria) are used in the Gram stain. If the positive control stains purple (Gram - positive result) and the negative control stains pink (Gram - negative result) as expected, it verifies that each step of the Gram - staining procedure (crystal violet application, iodine mordant, alcohol decolorization, safranin counter - stain) was done correctly. Options a, b, and c are not the main purposes of using positive and negative controls. Option a: Gram - variable bacteria are a different phenomenon not related to control usage. Option b: Determining an unknown's Gram type is the overall goal of the Gram stain, but controls are for verifying the procedure. Option c: Distinguishing Gram + and Gram - bacteria is the function of the Gram stain itself, not specifically the role of controls.

Answer:

d) To verify that the Gram stain was performed properly