Microbiology

Question 1

What shaped are the Coccus, Bacillus and Spirillum ?

Answer 1

Coccus = Spherical
Bacillus = Rod shaped
Spirillum = Spiral/ Corkscrew

Question 2

Outline the features of a gram positive cell

Answer 2

• Retains crystal violet stain appearing purple or violet
• Thick peptidoglycan (Murien) cell wall

Question 3

Describe the feature of a gram negative cell

Answer 3

• Stain pink or red with safranin
• Thin peptidoglycan wall
• Lipopolysaccharide layer is washed away with thealcohol

Question 4

Why is the lipopolysaccharide an important part of the gram negative cell?

Answer 4

Provides protection from attack by lysozymes and penicillin antibiotic. Therefore much more difficult to control in medical settings

Question 5

What is the method of gram staining?

Answer 5

1) Smear a glass slide with the bacterial sample and use heat to fix
2) Stain with crystal violet which binds to peptidoglycan
3) Treat with mordant Lugol’s iodine to fix the stain
4) Decoloursie with alcohol to remove unbound crystal violet stain
5) Counter stain with safranin

Question 6

How do you find the number of bacteria in the original sample?

Answer 6

Count the colonies on the plate and multiply by the dilution factor to find the number of bacteria in 1cm of the original sample.

Question 7

How can you count microbe reliably from an agar dish?

Answer 7

If there are between 20-200 colonies they are reliable, Colonies are not merged together to it is assumed each colony derived from 1 bacterium

Question 8

What does the aseptic technique prevent ?

Answer 8

Contamination of the environment by microbe being handles
Contamination of the cultures by microbes from the environment

Question 9

Describe the method of the aseptic technique

Answer 9

1) Petri dishes and agar should be sterilised before the agar is poured
2) Inoculating loop should be sterilised before and after use by red heat in the Bunsen Burner
3) Only lift Petroleum dish lid slightly
4) Petri dishes should be secured with tape and dated
5) Agar plates should be incubated at 25 degrees for 24-48 hours
6) Sterilised equipment after use

Question 10

What is a total count of microbes vs viable count?

Answer 10

Total count= living and dead cells
Viable count = living cells only

Question 11

What is an obligate aerobe?

Answer 11

Only reproduce or metabolise in oxygen rich environments

Question 12

What is a Facultative Anaerobe ?

Answer 12

Thrive in oxygen rich environments but also able to respire anaerobically

Question 13

What is an Obligate Anaerobe?

Answer 13

Unable to reproduce or metabolise when oxygen is present