Why is it important to use both positive and negative controls when performing a Gram stain?

• A. To verify that the staining procedure is working correctly and to ensure accurate interpretation of results
• B. To enhance the growth of Gram-positive bacteria
• C. To prevent contamination of the bacterial sample
• D. To increase the speed of the staining process

Answer: (A)

Verified step by step guidance

1. Understand the purpose of a Gram stain: it is a differential staining technique used to classify bacteria into Gram-positive or Gram-negative based on their cell wall properties.
2. Recognize that positive and negative controls are known samples used to validate the staining procedure. A positive control is a bacterium known to be Gram-positive, and a negative control is a bacterium known to be Gram-negative.
3. Using a positive control ensures that the staining reagents and procedure can correctly identify Gram-positive bacteria by showing the expected purple color.
4. Using a negative control ensures that the procedure can correctly identify Gram-negative bacteria by showing the expected pink/red color, confirming the decolorization step worked properly.
5. Together, these controls verify that the staining process is functioning correctly and help avoid misinterpretation of unknown samples due to procedural errors.