MFD6 (practical)

Question 1

1. Which component of bacterial cells retains crystal violet/iodine complexes during the Gram staining procedure?

Answer 1

thick peptidoglycan layer retains crystal violet/iodine complex in gram positive

Question 2

2. Which factor of Staphylococcus aureus is responsible for food poisoning?

Answer 2

Enterotoxin (note that this is a preformed toxin and that symptoms often occur within 4-6 hr or eating, which is much faster than infections that rely on growth).

Question 3

. Name three different characteristics/features of bacterial cells that can be helpful for identifying them by microscopy.

Answer 3

Shape/size, flagellae, pili, appendages, staining reactions, capsules, spores

Question 4

What type of substance is the target of diagnostic tests for Clostridium difficile?

Answer 4

spores

Question 5

Is Brilliance Candida agar a selective or an indicator medium?

Answer 5

Both! Note that Chloramphenicol inhibits bacteria. They don’t tell us the basis for the colour change apart from that it is ‘enzymes’ (trade secret!).

Question 6

Helicobacter pylori, the agent that causes stomach ulcers, produces urease enzyme. What benefit do you think H. pylori gets from urease production?

Answer 6

16s rRNA gene

Question 7

Helicobacter pylori, the agent that causes stomach ulcers, produces urease enzyme. What benefit do you think H. pylori gets from urease production?

Answer 7

allows it to metabolise urea into ammonia, which is alkaline , allowing it to raise the pH protecting it from stomach acid.`

Question 8

What is meant by the specificity of a diagnostic test?

Answer 8

The ability of the test to eliminate false positives.

Question 9

Which enzyme converts hydrogen peroxide to water and oxygen?

Answer 9

Catalase.

Question 10

What molecule interacts with staphylococcal clumping factor, and is used to coat latex beads in the coagulase test?

Answer 10

Fibrinogen.

Question 11

What change occurs in blood agar following the growth of β-haemolytic bacteria?

Answer 11

colour change from red to yellow

Question 12

What change occurs in blood agar following the growth of β-haemolytic bacteria?

Answer 12

the O-polysaccharide group forming LPS,

Question 13

Can you explain how the selective agents exploit knowledge of the natural habitat of the bacterial species?

Answer 13

Staphylococci live on the skin in sweaty places. They outcompete other human colonisers in the presence of sodium chloride. Enterobacteria, as their name suggests, live in the gut. Here, bile salts inhibit other organisms that are not adapted to live in that environment. Candida spp., as you might expect, can tolerate low pH. In the mouth, they thrive when dental plaque becomes acidic, as in the case of caries. They also live in the vagina, another low-pH environment. Candida spp. are fungi and are not sensitive to most classical antibiotics, which is why chloramphenicol is included as a selective agent.

Question 14

What species are selected by the following?

2) What are the selective agents:
a) staph agar
b) mcConkey agar
c) Candida agar

Answer 14

a) 1) staphylococci
2) sodium chloride
b) 1) enterobacteria
2) bile salts
c) 1) candida
2) low pH, chloramphenicol