BY4 - Microbiology

Question 0

What is gram stain?

Answer 0

A method of staining bacteria to help identify them

Question 1

Bacteria are classified into three main shapes. State them

Answer 1

Bacillus
Cocci
Spirillum
REMEMBER: get the spelling correct!

Question 2

How do most bacteria reproduce?

Answer 2

Binary fission

Question 3

What are the structural differences between gram negative bacteria and a gram positive bacteria?

Answer 3

Gram positive do not have a lipopolysaccharide layer whereas gram negative do.
Gram negative have a thinner peptidoglycan layer than gram positive.

Question 4

After gram staining how would a gram positive bacteria look? What causes it?

Answer 4

It would be purple because of the crystal violet.

Question 5

After gram staining how would a gram negative bacteria look?

Answer 5

It would appear red/pink

Question 6

Describe a gram stain process

Answer 6

All bacteria are stained with crystal violet.
The stain is fixed into cells using iodine.
Bacteria are decolourised using an alcohol.
A counter stain, such as safranin, is used.
Only the gram positive bacteria will retain the crystal violet and appear purple; whereas gram negative will not retain the crystal violet but will instead take up the counter stain and appear red/pink.

Question 7

Why are gram positive bacteria more susceptible to penicillin?

Answer 7

Because penicillin will interfere with the inter-peptide bonds within peptidoglycan making them weaker and prone to collapse. This means that when they attempt to divide the cell wall will disintegrate killing the bacteria.

Question 8

Why are gram negative bacteria resistant to penicillin?

Answer 8

Because of the lipopolysaccharide layer. They therefore require a different class of antibiotics.

Question 9

Name the two chemical components present in the lipopolysaccharide layer.

Answer 9

Protein and lipopolysaccharide.

This was part of a past paper question. WJEC June 2007

Question 10

Give the four conditions needed to culture bacteria

Answer 10

Nutrients: carbon in glucose; nitrogen in organic and inorganic form; mineral and salts.
Optimum temperature
Optimum pH
Oxygen: obligate aerobes require it, facultative anaerobes can survive without it but prefer it; obligate anaerobes are killed by it.

Question 11

What is a pathogen?

Answer 11

A disease causing microorganism

Question 12

What is a colony?

Answer 12

A cluster of bacterial cells which have arisen from one bacterial cell by asexual reproduction.

Question 13

What is a fermenter?

Answer 13

A vessel used to culture microorganisms, usually on an industrial scale

Question 14

Give the two methods of measuring growth

Answer 14

Total count: includes dead and living cells

Viable count: living cells only

Question 15

During serial dilution why is it important to have a sufficient dilution?

Answer 15

If insufficient the clumping of colonies will occur and counting is then inaccurate. This results in an underestimate of numbers.

Question 16

Define secondary metabolite and give an example.

Answer 16

A chemical that is not necessary for the growth of the fungus and is produced after the exponential growth phase is completed, when glucose is depleted.
Penicillin from Penicillium notatum.