Labset Two Worksheet

1. What is differential staining? How does it differ from simple staining? (2)
Differential staining uses more than one chemical stain. It can help differentiate between different microorganisms, or different parts of the cell. The simple stain uses one stain and is used to see cell shape and size.

2. What are the differences between gram positive and gram negative cell walls? (2) A gram positive has a thick wall of peptidoglycan and stains purple. A gram negative has a membrane covering the peptidoglycan and doesn’t stain leaving it a pink color.

3. What is a mordant? What serves as a mordant in the gram stain protocol? (2)

A mordant is a chemical that can deepen the reaction of the dye. Iodine is the mordant in the gram stain protocol.

4. Why do gram negative cells stain pink? Gram positive cells purple? (2)

Gram positive cells have cell walls and can retain the dye. Gram negative cells do not have cell walls and cannot retain the dye. Cristal Violet is added to the sample and penetrates the cell walls, staining the cell in the gram positive. Gram negative will not retain color as there is no cell wall. A counterstain is added to make the cell visible and turns it a pink.

5. What is measured by the methyl red portion of the MR-VP test? (2)

It measures acid in a bacterial broth. It turns red in pH under 4.4, yellow in pH over 6.2, and orange in between.

6. What is measured by the Voges-Proskauer portion of the MR-VP test? (2)
It detects acetoin in a bacterial broth culture. A red is positive for acetoin and yellow-brown is negative.

7. What is methyl red? Why is it included in the MR-VP broth? (2)
Methyl red is a dark red powder used as a pH indicator. By adding the methyl red, the pH level can be known.

8. Define the following terms: (2)

Monotrichous: