Microbiology 2

Question 1

How is media produced?

Answer 1

Either as liquids or solidified with agar

Question 2

Describe the characteristics of bacterial growth on agar plates.

Answer 2

• Grow in collection of cells called colonies
• Each colony arises from a single bacterium or a few bacteria
• Colonies can have diff forms, margins, elevations and colours

Question 3

What are the three main methods of counting bacteria?

Answer 3

1. Direct Counting method: microscopic count
2. Viable counting method: Standard plate count
3. Indirect Counting method: Turidity

Question 4

Describe the direct microscopic count method

Answer 4

The number of cells in a given volume of culture liquid is counted directly in 10-20 microscope fields by using a Petroff-Hausser chamber

Question 5

What is the advantage of the direct microscopic count?

Answer 5

Speed at which results are obtained.

Question 6

State the disadvantage of the direct microscopic count.

Answer 6

Often not possible to distinguish living from dead cells

Question 7

What does fluorescent dyes allow for in bacterial counting?

Answer 7

• Allows for simultaneous targeting of both live and dead cells

Question 8

Define a viable cell (standard plate count method).

Answer 8

A cell which is able to divide and form a population (colony)

Question 9

Describe the standard plate count method.

Answer 9

Usually done by diluting the original sample, plating aliquots of the dilutions onto an appropriate
culture medium, then incubating the plates under proper conditions so that colonies are formed.

Question 10

Describe the pour plate method for standard plate count.

Answer 10

• Start with a serial dilution of the culture
• Allows for more sensitivity so is ideal for samples with lower bacterial numbers

Question 11

Describe the spread plate method for standard plate count.

Answer 11

• Starts with a serial dilution of the culture
• Simple, easy and quick

Question 12

Describe the indirect method - turbidity.

Answer 12

As the numbers of bacteria in a suspension increase, the turbidity also increases and causes less light to reach the detector
- A spectrophotometer is used to measure cloudiness of bacterial cell suspension as an indirect measure of cell density
- Standard curve can then be drawn

Question 13

Why do we test the effectiveness of antimicrobials?

Answer 13

To determine the most appropriate antimicrobial agent to use against an infection

Question 14

What are the two most commonly employed techniques to test the effectiveness of antimicrobials?

Answer 14

1. Tube dilution assay
2. Disc diffusion method

Question 15

Describe the tube dilution assay.

Answer 15

• The test organism is incubated with
  serially diluted antibiotic.
• The lowest concentration capable of
  preventing microbial growth is the MIC.

Question 16

Describe the disc diffusion method.

Answer 16

• The bacterium to be tested is spread on an agar plate, then paper discs
  impregnated with appropriate antibiotics are placed on the surface.
• Following incubation, susceptibility to an antibiotic is indicated by a clear ring
  surrounding the disc.

Question 17

Why do microbial growth media need to be sterilised?

Answer 17

• To prevent contaminants from growing in them

Question 18

What are the two sterilisation methods?

Answer 18

1. Physical methods
2. Chemical methods

Question 19

Describe sterilisation by heat.

Answer 19

• Boiling at 100 degrees celsius for 10 minutes
• Most bacteria are killed at about 70 degrees however if endospores are present they can resist boiling for several hours

Question 20

Describe moist heat sterilisation.

Answer 20

• Performed by steam under pressure: autoclaves
• Autoclaves remove air from the chamber and replace it with pure saturated steam

Question 21

What are the two chemical sterilisation methods?

Answer 21

• Gaseous sterilisation and Liquid sterilisation