6.1 Microbial techniques

Question 1

What is provided in a culture?

Answer 1

Correct nutrients, oxygen, pH, temperature

Question 2

What is aseptic technique?

Answer 2

Only introducing the desired bacteria into the medium, under sterile conditions to prevent the growth of unwanted organisms

Question 3

Factors to consider when culturing a microbe

Answer 3

• Even a harmless microbe has a risk of a harmful mutant strain
• Contamination from the air or your skin

Question 4

Describe 3 types of culture

Answer 4

Agar plate - nutrient agar, which is a solid medium that acts as a sponge to hold nutrients in a solid block
Selective/differential media - A recipe that discourages the growth of unwanted microbes, relying on the addition or omission of a certain nutrient or include an antibiotic to which the desired bacterium is resistant to
Broth cultures - Nutrients in liquid form

Question 5

Describe a cell count as a method of measuring growth

Answer 5

• Cells can be counted using a haemocytometer
• Broth is diluted 1:1 with trypan blue (which stains dead cells)
• Count the bacterial cells in each of the four sets of 16 squares and take a mean

Question 6

Benefits of cell counting

Answer 6

Useful and only counts viable cells

Question 7

Problems with cell counting

Answer 7

Slow and equipment is expensive

Question 8

Describe turbidimetry

Answer 8

Specialised form of colorimetry
- As turbidity increases, absorbance increases
- By measuring samples with a known cell count and creating a calibration graph the cell count in the unknown sample can be obtained

Question 9

Benefits of turbimetry

Answer 9

Quick and can be conducted in the field

Question 10

Problems with turbimetry

Answer 10

Expensive, easily affected by other variables, counts non-viable cells, assumes that density is equal across the culture

Question 11

Describe dilution plating

Answer 11

Works on the principle that every colony is grown from a single, viable microorganism
- Culture is serially diluted because they are usually a single mass
- A lawn plate is made and the colonies are counted
- This is them multiplied by the dilution factor to obtain a cell count

Question 12

Benefits of dilution plating

Answer 12

• Doesn’t require complex or expensive equipment
• Only counts viable cells and obtains a direct cell count

Question 13

Problems with dilution plating

Answer 13

Slow because it requires an incubation period and serial dilutions are required

Question 14

What are the phases of bacterial growth

Answer 14

Lag - adaptation to the environment
Log - maximum rate of growth
Stationary - death rate = reproduction rate due to build up of waste and lack of nutrients
Death - Death rate exceeds as conditions deteriorate