Introduction

Question 1

What are the features of prokaryotes?

Answer 1

1. size (bigger than viruses, smaller than eukaryotic cells)
2. single cell
3. cell wall
4. asexual reproduction (binary fission)
5. lateral transfer of genetic material
6. circular chromosome (usually)
7. lack of nucleus or other membrane bound organelles
8. unique metabolic requirements / abilities

Question 2

What is a gram stain?

Answer 2

• based on structure of cell wall
• Gram (+) cell wall has a thick layer of peptidoglycan which traps dye and prevents it from being washed away (appears purple) (peptidoglycan can be 40-80% of dry weight)
• in gram negative, outer membrane is an important barrier to many microbials (appears pink)
• gram stain reactions are a fundamental description of bacteria

Question 3

How is a gram stain performed?

Answer 3

Slides are treated with:
1. crystal violet (primary stain)
2. gram’s iodine
3. acetone (decolourizer)
4. Carbol fuchsin or Safronin (counter stain)
- crystal violet/iodine crystal complexes get stuck in think gram (+) peptidoglycan & resist decolourization
- gram (-) are decolourized & subsequently counter stained

Question 4

What features are used to classically describe bacterial species?

Answer 4

• gram-stain characteristic
• cell morphology
• atmospheric requirements
• biochemical/metabolic characteristics

Question 5

BIG PICTURE: Describe the differences between the Gram-positive & Gram-negative cell wall with respect to the following components: peptidoglycan, membranes, porins, and the periplasmic space

Answer 5

• Gram (+) have much thicker peptidoglycan layer which will trap primary dye
• gram (-) have outer membrane (2 layers), only 1 membrane in gram (+)
• porins are found in gram (-) & allow things to pass though (not in G (+))
• gram (-) have periplasmic space, but gram (+) do not