Diagnosis and Tests

Question 1

What is the basic agar recipe?

Answer 1

Water
Protein derivative
Yeast extract
Gelling agent
Buffer

Question 2

What is enriched non-selective agar?

and give 1 example

Answer 2

It grows of most bacteria (some exceptions)

e.g. blood agar

Question 3

What is selective agar?

and give 1 example

Answer 3

It contains an agent to prevent the growth of specific microbes / only allows selective microbes to grow, inhibiting the growth of all others

e.g. mannitol salt

Question 4

What is differential?

and give 2 example

Answer 4

It contains a means of differentiating between organisms on the media

exploits the biochemical properties of different microbes to help distinguish one strain from another

e.g. MacConkey (LF-pink, NLF colourless), chromagen(ic)

Question 5

What is enrichment agar?

and give 1 example

Answer 5

Supports the growth of the target organism before subculture onto agar plates, often in the form a broth
e.g. Selenite

Question 6

What is chromagenic agar?

and what 2 other media does it act as?

Answer 6

It contains chromogens often linked to specific sugars, will change the colour of the colonies to aid in identification
Acts as selective and differential

Question 7

What is specialised agar?

and give 1 example

Answer 7

e.g. MacConkey sorbitol agar

Question 8

What is blood agar?

Name 2 organism that grows well and 1 that doesn’t?

Answer 8

It contains blood, and used to detect haemolytic activity
Grows well - Staphylococcus, streptococcus
Doesn’t

Question 9

What is chocolate blood agar?

Name 1 organism that grows well and 1 that doesn’t?

Answer 9

Contains lysed blood
Grows well: H. influenzae
Doesn’t:

Question 10

What are 2 advantages of chromagenic agar?

Answer 10

1. can put multiple samples on the same plate

2. can identify mixed culture, due to difference in appearance

Question 11

What is CLED culture media?

Answer 11

Cystine lactose electrolyte deficient, identify lactose fermenters vs non-lactose fermenters

Question 12

What is a disadvantage of this CLED media?

Answer 12

It can be difficult to differentiate between colony types

Question 13

What is the 3 types of cell culture?

Answer 13

primary, semi-continuous, continuous

Question 14

How is a Gram stain performed?

Answer 14

1. crystal violet - initial dye
2. iodine - dye trapped
3. alcohol - discolouration
4. safranin - alternative dye

Question 15

What is the difference between positive and negative Gram stain?
Give 2 examples of each

Answer 15

+ve: thick peptidoglycan layer, e.g. s. aureus, b. subtilis

-ve: thin peptidoglycan layer and also have an additional, lipid rich outer membrane, e.g. e. coli, p. aeruginosa

Question 16

What is a ZN stain?

Answer 16

Ziehl-Neelsen stain, used to identify acid fast organisms

Question 17

What does a positive and negative ZN stain look like?
Give 1 example for each result
Identify an image of the stain

Answer 17

+ve: pink, acid fast, e.g. Mycobacterium tuberculosis
-ve: blue, non-acid fast
(blue background, pink long rods)

Question 18

How is a ZN stain performed?

Answer 18

1. primary stain - pink
2. heat - mordant
3. alcohol - discolouration
4. methyl blue - counter stain

Question 19

Give 2 functions of a diagnostic microbiology lab

Answer 19

1. Identification of a causative agent

2. Specific organism reporting

Question 20

What does the Oxidase test show

and what does a positive and negative result look like

Answer 20

Aerobic/anaerobic
Culture rubbed on oxidase soaked filter paper
+ve: immediate purple
-ve: not immediate

Question 21

What does the Catalase test show

and what does a positive and negative result look like

Answer 21

Production of catalase
Hydrogen peroxide added to culture
+ve: bubbles
-ve: no bubbles

Question 22

How are antibiotics tested (if bacteria is resistant or not)

Answer 22

Disc diffusion method

Question 23

Give an example of a +ve and -ve Oxidase test

Answer 23

+ve: P. aeruginosa

-ve: E. coli (and S. aureus)

Question 24

Give an example of a +ve and -ve Catalase test

Answer 24

+ve: S. aureus (and B. subtilis, P. aeruginosa)
-ve: S. pneumoniae
strep vs staph

Question 25

Give an example of +ve and -ve Coagulase test

Answer 25

+ve: S.aureus

-ve: coagulase -ve staphylococci (CNS)

Question 26

Name 3 reason why aseptic techniques are important

Answer 26

1. Prevent of microbial infection of users
2. Prevent contamination of local environment
3. Prevent production of false results due to specimen contamination

Question 27

Give 3 aseptic techniques

Answer 27

1. Organising a clean work surface with all necessary materials to hand
2. Flame inoculation loop before and after transferring to medium
3. Being careful not to touch the sides and working quickly, next to the flame, use the inoculation loop to retrieve the specimen

Question 28

Give 2 benefits of using agarose

Answer 28

1. It is inert

2. It only melts at high temperatures, so remains solid at incubation temperatures

Question 29

Why is it important to store agar plates with the gel at the top?

Answer 29

As colonies grow they produce vapour and condensation can quickly begin to accumulate. Storing plates ‘upside down’ prevents this moisture from contaminating and mixing up individual colonies.

Question 30

Which is the most critical step of the gram stain and why?

Answer 30

The acetone - If left too long, all cells become gram negative.

Question 31

Give the 5 steps to complete a risk assessment

Answer 31

1. identify hazard
2. decide who might be harmed
3. evaluate the risk
4. record findings
5. renew risk assessment and update