Biology: 3.4 Microbiology

Question 1

What is murein?

Answer 1

It is a peptidoglycan: molecules of polysaccharide, cross linked by amino acids.

Question 2

What is a lipoprotein layer?

Answer 2

This layer can protect bacteria from antibiotics and from enzymes.

Question 3

How is the gram stain a double stain?

Answer 3

The main stain - crystal violet - reacts with and stains the murein cell wall purple.
If there is a lipoprotein coat the stain can be dissolved out of the layer using ethanol and prevents the crystal violet from getting to the murein cell wall.
The counter stain - safranin - reacts with the lipoprotein coat and stains it red.

Question 4

What colour is gram positive bacteria after staining it?

Answer 4

Purple

Question 5

What colour is gram negative bacteria after staining it?

Answer 5

Red

Question 6

Explain why the gram negative wall can’t retain the crystal violet stain.

Answer 6

Gram positive has a thick murein wall which retains the gram stain.
Gram negative has a thin murein wall and there’s little murein, lipoprotein coat prevents the crystal violet gram stain from getting to murein cell wall.

Question 7

What are obligate aerobes?

Answer 7

This type of bacteria can grow only in the presence of oxygen

Question 8

What are obligate anaerobes?

Answer 8

This type of bacteria can only grow if there is no oxygen present.

Question 9

What are facultative anaerobes?

Answer 9

This type of bacteria thrive in environments with or without oxygen although grow best in presence of oxygen.

Question 10

What are the basic requirements for culturing bacteria?

Answer 10

1: A suitable temperature
2: A suitable pH
3: Water
4: Oxygen or no oxygen ( depending on the type of bacteria)
5: Energy / nutrients

Question 11

Why do bacteria require carbon?

Answer 11

It’s needed for making organic compounds of the bacterium.

Question 12

Why do bacteria require a nitrogen source?

Answer 12

It’s needed for making proteins and nucleic acids.

Question 13

How can bacteria be cultured?

Answer 13

1: On solid growth media - eg agar plates.
2: In liquid culture - eg nutrient broth.

Question 14

What do aspectic techniques reduce the risk of?

Answer 14

1: Contamination of the bacterial culture
2: Potential culture of pathogenic bacteria
3: Contamination and possible infection of personnel handling the bacteria
4: Contamination of the immediate environment and infection of other people.

Question 15

How do you make sure sterilisation happens?

Answer 15

All equipment must be sterile.
1: Inoculating loops - hold in Bunsen burner flame before and after each use.
2: Petri dishes - Sterilised by irradiation with gamma rays.
3: Culture medium - Agar is heated in an autoclave - this ensures that heat resistant bacteria and spores are killed.
4: Air supply: Air supply filtered to reduce contamination. (ensure O2 reaches the culture so not to grow anaerobic human pathogens.)

Question 16

Why is it important that we limit exposure time of the agar to the air?

Answer 16

To reduce the risk of contamination by air-borne spores and also to reduce the risk of the microbe into the air.
Once Petri dishes are inoculated fix the lid.

Question 17

What assumption is made when culturing bacteria?

Answer 17

Individual colony has grown from a single viable bacterium. (1 colony = 1 bacterium.)

Question 18

What does the estimate of culturing bacteria not allow for?

Answer 18

Doesn’t allow for clumping and therefore is likely to give an underestimate of the actual number of viable cells in the sample.