Microbiology (DONE)

Question 1

What are the 3 shapes (and their names) that are used when classifying bacteria? How is shape maintained

Answer 1

• Spirals =>Spirrila/Spirrilium
• Rods => Bacilla/Bacillus
• Spherical => Cocci/Coccus
• Rigidity of cell wall

Question 2

Describe the differences between gram positive and negative bacteria and what colour each bacteria will be stained.
(6 marks)

Answer 2

Gram +ve
- Thick layer of peptidoglycan
- No lipopolysaccharides
- Stained purple by crystal violet as violet/iodine complex is retained within the cell

Gram -ve
- Thinner layer of peptidoglycan
- Outer lipopolysaccharide later.
- Stained pink/red by safranin as purple stain complex is washed off when treated with alcohol.

Question 3

Describe the gram staining procedure.
(4 marks)

Answer 3

1. Crystal violet
2. Iodine
3. Alcohol (dehydrates the peptidoglycan)
4. Safranin (counter stain)

Question 4

To culture bacteria in a laboratory, what must the agar jelly media include and what conditions should it be at?
(6 marks)

Answer 4

• Carbon source (glucose/lactose)
• Nitrogen source (ammonia/amino acids)
• Sulphur and phosphorus
• Vitamins and minerals
• Suitable pH
• Suitable temperature

Question 5

What are the definitions of the following?
- Obligate aerobes
- Obligate anaerobes
- Facultative anaerobes

Answer 5

• Obligate aerobes -> Growth is inhibited in the absence of oxygen/Need oxygen for respiration
• Obligate anaerobes -> Growth is inhibited in the presence of oxygen/Only survive in the absence of oxygen
• Facultative anaerobes -> Can grow in both the presence and absence of oxygen, but grow best in the presence of oxygen

Question 6

What are the four main requirements for bacterial growth?

Answer 6

• Nutrients
• Moisture
• Temperature
• Time

Question 7

What is aseptic technique used to prevent?
(2 marks)

Answer 7

• Contamination of the environment by the microorganisms
• Contamination of the bacterial culture by unwanted microorganisms from the environment

Question 8

How can sterilisation/aseptic technique be achieved?
(5 marks)

Answer 8

• Pass inoculating loop through a roaring blue Bunsen flame until the loop glows red
• Pre-sterilised petri dishes
• Autoclave glassware/equipment with high pressure and high temperature (121)
• Pass the mouth of the bottle containing bacteria through flame for gases to expand
• Heat labile plastics are irradiated

Question 9

What are the two types of counts used for counting microbes and what is counted in both?

Answer 9

• Viable count (living cells only)
• Direct/Total count (both living and dead)

Question 10

What assumptions are made when counting cells?
(2 marks)

Answer 10

• One cell gives rise to one colony
• No allowance for clumping of cells so may cause an underestimate of numbers

Question 11

Give a method for using serial dilution for the viable count method.

Answer 11

• Serial dilution of stock 0.1 or 0.01
• Add 0.5/1.0 ml to agar plate
• Incubate at 25 for 24/48 hours
• Count colonies
• Use aseptic technique

Question 12

What rule is used when using a haemocytometer?

Answer 12

North-west rule