Microbiology

Question 1

Name the four shapes of bacteria and their appearance

Answer 1

• Bacillus, rod shaped
• Coccus, spherical/oval shaped
• Spirillum, corkscrew-shaped rods
• Vibrio, comma-shaped

Question 2

How is a Gram stain carried out?

Answer 2

• Crystal violet added to stain all bacteria purple
• Iodine added to allow the purple stain to further bond to Gram-positive bacteria
• Acetone-alcohol added to remove the purple stain from Gram-negative bacteria
• Safranin added to stain Gram-negative bacteria red

Question 3

What is different between the cell walls of Gram-positive and Gram-negative bacteria?

Answer 3

• Gram-positive has a thick layer of peptidoglycan which the purple stain stays bonded to
• Gram-negative have an outer layer of lipopolysaccharide which prevents the purple stain from properly bonding

Question 4

Which type of bacteria is harder to treat with bacteria and why?

Answer 4

• Gram-negative

- Because of the outer lipopolysaccharide layer

Question 5

Why is aseptic technique carried out?

Answer 5

• To prevent contamination of the environment by microbes being handled
• To prevent contamination of the cultures by unwanted microbes from the environment

Question 6

Name some ways in which aseptic technique should be carried out

Answer 6

• Sterilise all work surfaces before and after with a disinfectant (e.g. 3% Lysol)
• Work near a lit Bunsen burner which creates an updraft
• Do not place lids of bottles down and keep them in hand
• Flame mouths of bottles before and after use
• Flame inoculating loop (if used) until red hot and allow to cool in air
• Sterilise all equipment before and after use either via an autoclave or via gamma irradiation

Question 7

What are the requirements for growth for bacteria?

Answer 7

• Carbohydrates (for respiration)
• Oxygen (or the lack of)
• Mineral ions
• pH (a suitable one)
• Vitamins
• Energy source (could be carbohydrates)
• Nitrogen (a source of)
• Temperature (a suitable one)
• Water

Question 8

Name the three ways of measuring the size of a bacteria population

Answer 8

• A total cell count (living and dead cells)
• A viable count (only living cells)
• Using a colorimeter to measure turbidity/cloudiness

Question 9

How is the number of bacteria in an original culture worked out when using a serial dilution?

Answer 9

• Divide the number of colonies by the volume of solution placed of agar
• And multiply by the dilution factor

Question 10

When is a plate unsatisfactory for counting the number of colonies?

Answer 10

• When there is an unreliably small sample (generally less than around 30 colonies)
• When there is too many colonies so they overlap (generally more than around 100)