Question

why do we use positive and negative controls in the gram stain? a) to distinguish between gram - positive and gram - negative bacteria b) to exclude gram - variable bacteria c) to determine if an unknown sample is gram - positive or negative 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) when the Gram stain is performed, it indicates that the staining procedure was carried out correctly. If these controls do not show the expected results, there may be an error in the staining process (e.g., incorrect reagents, improper timing of steps like decolorization).

• Option a: Distinguishing between Gram - positive and Gram - negative bacteria is the general purpose of the Gram stain itself, not specifically the role of positive and negative controls.
• Option b: Excluding Gram - variable bacteria is not the main function of positive and negative controls. Gram - variable bacteria have inconsistent staining results due to their own characteristics (e.g., some bacteria in a culture may have damaged cell walls), and controls do not directly address this.
• Option c: Determining if an unknown sample is Gram - positive or negative is the result of staining the unknown sample. The controls are to check the validity of the staining process, not to directly determine the unknown's Gram status (although a properly done stain with working controls helps in accurately determining the unknown's status).

Answer:

d) To verify that the Gram stain was performed properly