Component 1.4 - Classification Of Bacteria

Question 1

What are the 3 different shapes of bacteria and how is this portrayed in their name?

Answer 1

Bacillus/rod- shaped e.g Escherichia Bacillus
Coccus or spherical e.g Staphylococcus
Spiral or corkscrew-shaped e.g Spirilium

Question 2

What is the shape of bacteria determined by?

Answer 2

Its cell wall contains a 3D mesh of peptidoglycan (murein). The cross-linking between these molecules provides strength and gives the cell its shape.

Question 3

What colour are gram-negative and gram-positive bacteria after staining?

Answer 3

Gram-positive bacteria are stained violet

Gram-negative bacteria are red

Question 4

Why are Archaea and Eukaryotes not stained by Gram stain?

Answer 4

Gram stain binds to the peptidoglycan in bacterial cell walls. Archaea and Eukaryotes do not contain peptidoglycan.

Question 5

What happens when crystal violet is added to gram-positive and gram-negative bacteria? (1)

Answer 5

Binds to peptidoglycan so all bacteria stain purple

Question 6

What happens when alcohol is added to gram-positive and gram-negative bacteria after crystal violet? (2)

Answer 6

Gram-negative bacteria lose their colour and become colourless
Gram-positive bacteria remain purple

Question 7

Why does gram-positive bacteria stay the same colour when washed with alcohol?

Answer 7

Gram positive bacteria have cell walls with a thicker layer of peptidoglycan/murein (than Gram-negative bacteria), which retains the crystal violet/iodine complex within their cells

Question 8

What happens to the cell wall of gram-negative bacteria when they are washed with alcohol?

Answer 8

Their cell walls lose their outer lipopolysaccharide membrane, and the thin inner peptidoglycan layer is left exposed, this means that the crystal violet/iodine complexes are washed from the gram-negative cell along with the outer membrane.

Question 9

What colour are gram-negative bacteria stained with the counter stain safranin? (3)

Answer 9

Red (gram-positive are purple)

Question 10

How does penicillin and lysozyme effect gram-positive bacteria?

Answer 10

Lysozyme - hydrolyses bonds holding peptidoglycan together

Penicillin - prevents inter-linking peptidoglycan molecules from forming. Water uptake by osmosis bursts the cell.

Question 11

How do penicillin and lysozyme effect gram negative bacteria?

Answer 11

Due to them having a layer of lipopolysaccharide protecting their peptidoglycan they are resistant to penicillin (cannot penetrate cell wall) and lysozyme has no effect on them