Bacterial Classification by Shape and Gram Stain

Overview

Recall questions on Bacterial Classification by Shape and Gram Stain.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Name the four shapes used to classify bacteria and describe each.

   Model answer

   Cocci are spherical, bacilli are rod-shaped, spirilla are spiral or corkscrew-shaped and vibrios are curved like a comma.

2. 2

   Distinguish between streptococci and staphylococci.

   Model answer

   Streptococci are spherical cells joined in chains, whereas staphylococci are spherical cells in irregular, grape-like clusters.

3. 3

   Which feature of a bacterium determines whether it is Gram positive or Gram negative?

   Model answer

   The structure of its cell wall, in particular the thickness of the peptidoglycan layer and whether there is an outer lipopolysaccharide membrane.

4. 4

   Describe the four staining steps of the Gram technique in order.

   Model answer

   The heat-fixed smear is flooded with crystal violet, then with iodine solution, then decolourised briefly with ethanol, and finally counterstained with safranin.

5. 5

   Why do Gram positive bacteria retain the crystal violet?

   Model answer

   Their thick peptidoglycan layer traps the large crystal violet–iodine complex, which the ethanol cannot wash out, so the cells stay purple.

6. 6

   Why do Gram negative bacteria lose the crystal violet?

   Model answer

   Ethanol dissolves the lipid-rich outer membrane and the thin peptidoglycan layer cannot hold the dye complex, so it is washed out and the cell takes up the red counterstain instead.

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.