3.4 Microbiology

Question 1

What are bacterial cell walls made up of?

Answer 1

Peptidoglycan called murein, a polymer of amino acids and sugars

Question 2

What is gram staining?

Answer 2

A technique used to differentiate between gram negative and gram positive bacteria

Question 3

Outline the process of gram staining.

Answer 3

1. Stain culture with crystal violet. Rinse with water
2. Add iodine solution and rinse and 1 minute
3. Alternate washes of alcohol and water for 30 seconds
4. Counterstain with red safranin for 1 minute
5. Dry and examine sample under microscope

Question 4

Define gram positive bacteria

Answer 4

Bacteria that have a thick peptidoglycan wall and a purple appearance following gram staining

Question 5

Why do gram positive bacteria appear purple following gram staining

Answer 5

The thick peptidoglycan wall retains crystal violet when rinsed with alcohol

Question 6

Define gram negative bacteria

Answer 6

Bacteria have a thin peptidoglycan wall with an outer lipopolysaccharide membrane and a red appearance following gram staining

Question 7

Why do gram negative bacteria appear red following gram staining

Answer 7

When alcohol is used, the lipopolysaccharide layer is lost and the crystal violet washes away. The counterstain safranin stains the thin peptidoglycan layer red.

Question 8

What is an obligate aerobe?

Answer 8

An organism that requires oxygen for metabolism

Question 9

What is an obligate anaerobe?

Answer 9

An organism that can only survive in environments which lack oxygen

Question 10

define facultative anaerobe

Answer 10

An organism that normally respires aerobically, but it is capable or switching to anaerobic respiration in absence of oxygen

Question 11

What are aseptic techniques ?

Answer 11

A range of techniques used to culture microorganisms under sterile conditions in order to minimise contamination

Question 12

List the basic aseptic techniques

Answer 12

• wipe surfaces with antibac
• set up Bunsen burner nearby- convection currents prevent microbes from entering culture
• flame the inoculating loop
• sterilise all equipment
• wear protective clothing

Question 13

What is the difference between total cell count and viable cell count

Answer 13

In a given area or volume, total cell count is the total number of cells living and dead whereas viable is only living cells.