C1: Microbiology

Question 1

What are the three main shapes of bacteria

Answer 1

Rod shaped: Bacilli (plural), Bacillus (singular)

Spiral: Spirilla (plural), Spirillum (singular)

Spherical : Cocci (plural) , Coccus (singular)

Question 2

What is the test to distinguish between gram negative and gram positive bacteria

Answer 2

Gram staining - a method of staining the cell walls of bacteria as an aid to their identification

Question 3

What colour are gram positive and gram negative bacteria after gram staining

Answer 3

Gram positive - Purple
Gram negative - Red/Pink

Question 4

Describe the stucture of Gram positive bacteria

Answer 4

Thick peptidoglycan (murein) cell wall
Cell membrane

Question 5

Describe the stucture of gram negative bacteria

Answer 5

Lipopolysaccharide layer
Thin Peptidoglycan cell wall
cell membrane

Question 6

Describe the process of Gram staining

5 points

Answer 6

Smear a glass slide with bacterial sample and use heat to fix

Add Crystal violet - binds to gram + peptidoglacan layer

Add iodine - To fix the stain to peptidoglycan

Add Ethanol - removes excess stain and removes the lipopolysaccharide layer of gram -

Add safranin - counter stain, dyes gram - Red/Pink

Question 7

Explain the role of the lipopolysaccharide layer.

Answer 7

Protects gram negative bacteria
eg: is not susceptable to penecillin and lysozyme

Question 8

Explain how penecillin and lysozyme are antibiotics

Answer 8

Penecillin - prevents inter-linking bonds between peptidoglycan molecules from forming this weakens the cell wall and they can burst with the uptake of water

Lysozyme - hydrolyses the bonds holding the peptidoglycan molecules together

Question 9

Conditions for growth of microbes

6 points

Answer 9

Carbon source - generally glucose

Nitrogen source - needed for the synthesis of nitrogenous compounds

Growth factors - such as water, vitamins and mineral salts

Temperature - But should not be grown at 37C in order to not grow pathogens

pH - optimum should be considered

Oxygen :
Obligate aerobes - Can only grow or metabolise in the presence of oxygen

Faculative anaerobes - Thrive in an oxygen rich enviroment but can grow in an anaerobic enviroment also

Obligate anaerobe - Unable to grow or metabolise where there is oxygen present

Question 10

Name the different culture media

4 points

Answer 10

Defined medium - known ingredients only

undefined medium - not all components are known

a selective medium - only allows certain bacteria to grow

a complete medium - contains all the chemicals required for growth

Question 11

Define aseptic technique

Answer 11

Lab technique practice that maintains sterility in apparatus and prevents contamination of the equipment and the enviroment

Question 12

What does asceptic technique aim to prevent

2 points

Answer 12

contamination of the environment by the microbes being handled

contamination of microbial cultures by unwanted microbes from the environment

Question 13

How do you sterilise equipment

2 points

Answer 13

Sterilise at 121C for 15 mins in an autoclave
or
Pass inoculating loop and tops of tubes over a bunsen burner flame

Question 14

Define pathogen

Answer 14

An organism that causes disease in its host

Question 15

What is the difference between total cell count and viable cell count

Answer 15

Viable count - counting colonies and are living cells only

Total cell count - (Uses a haemocytometer) is both living and dead cells

Question 16

Define colony

Answer 16

A cluster of cells, or clone, which arises from a single bacterium or fungal spore by asexual reproduction

Question 17

What is the assumption made when plating and counting colonies and how might this effect the overall estimation of the viable count

Answer 17

One counted colony is derived from one living cell

excludes the possibility of clumping so the result may be an underestimate

Question 18

Why is serial dilutions necessary

Answer 18

It is not possible to count as the population density is too high and therefore a dilution is necessary to have a countable range with distinguished colonies

Question 19

Describe the process of serial dilutions

Answer 19

1cm^3 of culture is added to a test tube with 9cm^3 of medium to form a 10^-1 dilution
1 cm^3 of this dilution is then used and added to 9cm^3 of medium to form a 10^-2 dilution

this process is repeated until a usable dilution is acheived

Question 20

What is the ideal number of colonies in a petri dish after serial dilutions to give a reliable estimate

Answer 20

20-100 colonies that are distinct and seperate