6.1 Microbial Techniques

Question 1

What are the two forms of growth mediums

Answer 1

• Nutrient Broth
• Nutrient Agar

Question 2

Why are pour plates used ?

Answer 2

• Used to perform viable plate counts
• Which lets us generate growth curves

Question 3

What is the technique of pour plates

Answer 3

• ## Molten agar inoculated before it solidifies at around 50-45c.

Question 4

What is the result of a pour plate

Answer 4

Colonies distributed throughout the solid medium.

Question 5

Why are spread plates used ?

Answer 5

• Used to count number of microorganisms within the sample
• Used to help identify microorganisms on how the colonies look

Question 6

What is the technique of spread plates ?

Answer 6

• Culture is uniformly spread over an agar plate
• If not diluted, a lawn of bacteria can arise

Question 7

What is the result of the agar plate

Answer 7

Isolated colonies distributed across the agar plate if diluted correctly

Question 8

Why is a streak plate used

Answer 8

To obtain a pure culture from a mixed culture

Question 9

What is the technique of a streak plate

Answer 9

• Inoculating loop is used to add streaks of a culture onto the plate
• Plate is quarter turned after each streak set
• Each new direction , new streak is passed through old streak

Question 10

What is the result of a streak plate

Answer 10

• At the end of the streak, individual colonies should be able to be identified and removed from the plate

Question 11

Why is it important ASeptic Technique is used when carrying out salmonella rapid culture technique

Answer 11

• Public health workers may become contaminated with salmonella
  -Cross contamination could lead to false identification of bacteria

Question 12

Describe how you would use aseptic techniques to transfer bacterial cells growing on an
agar plate to a tube containing a sterile broth.

Answer 12

• Disinfect surfaces
• Work near a bunsen flame
• Flame top of the tube
• Heat the wire loop
• Open the petri dish to a small extent

Question 13

What are the different phases of a bacterial growth curve

Answer 13

• Lag phase
• Log phase
• Stationary phase
• Death Phase

Question 14

Explain the lag phase

Answer 14

• The microorganisms are adapting to their environment and reproduction rate increases slowly

Question 15

Explain the log phase

Answer 15

• Microorganisms will grow at their maximum rate as long as there is sufficient nutrients

Question 16

Explain the stationary phase

Answer 16

• Death Rate = reproduction rate
• Due to lack of nutrients

Question 17

Explain the Death phase

Answer 17

• Where the deaths exceed new population

Question 18

How can cell counts be carried out ?

Answer 18

• Via using a haemocytometer

Question 19

Describe how the haemocytometer can be used for a cell count

Answer 19

• Contains 4 cell grids
• Small

Question 20

Explain Turbidity

Answer 20

• Special form of colorimetry
• As the number of bacteria cells in a culture increases, the culture becomes more cloudy.
• As the solution becomes more cloudy, less light passes through

Question 21

Evaluate the use of dilution plating and optical methods for determining the number of bacterial cells in a culture (6)

Answer 21

• Dilution plating takes longer due to incubation period, optical methods are quicker
• Dilution plates only counts living cells however optical methods measure living and dead cells
• Dilution plating gives direct count but optical methods give turbidity reading
• Diluting plating requires easily available apparatus but optical methods require colourimetry
• Dilution plating - error occurs if colonies run into each other
  Optical method - Error if tube not shaken

Question 22

Why is spread plating a suitable method for isolating Salmonella ?

Answer 22

• G Medium only allows salmonella to grow
• Culture is spread out on the medium
• Because this separates the medium
• So that colonies are individual and can be picked up