A Gram stain is a dye based microscopic technique that allows us to determine if our bacteria is of a single type and if it is the correct shape and color for the species we are working with. All bacteria have a distinct morphology that allows us to determine their type.

The two main groupings in bacteria are gram positive and gram negative. Gram positive bacteria have an extra outer layer of peptidoglycan that will retain the blue dye in the gram staining procedure and therefore appear blue or purple when observed under the microscope. Gram negative bacteria do not have this peptidoglycan layer and therefore do not retain the blue dye and will then appear red or pink under the microscope. These are the two main categories of bacteria, Gram positive and Gram negative.

The other thing a gram stain can show us is the shape of the bacteria. Bacteria come in three major shapes; spheres, rods and spirals.

Once the Gram stain is performed correctly we can tell if we have a pure monoculture and what type of bacteria we have cultured. This is very important because bacteria are the starting material for many of our biotechnology experiments and if this step is not correct then everything downstream will be incorrect also.

The Gram stain procedure:

Gram Staining

Read this peer-reviewed journal for more information.

Remember: The next step is to come into the lab to practice with the materials, view a demonstration, or to even just ask questions. Also, if you are ready to take the lab final, please inform your instructor. The lab final will be performed by yourself and will be graded with a rubric.

### Learning Objectives:

- 1.5 - Complete a gram stain.
- 1.6 - Describe the bacteria you observe in your gram stain.