Microbiology (C1)

Question 1

What determines the shape of a bacteria, and what is it composed of?

Answer 1

the shape of bacteria is due to their rigid cell wall which has a unique structure - it contains a 3D mesh of
peptidoglycan

Question 2

Describe the structure of gram positive bacteria and its staining colour?

Answer 2

Gram positive bacteria have cell walls with no outer lipopolysaccharide layer and with a thicker layer of peptidoglycan (than Gram-negative bacteria), which retains the crystal violet/iodine complex within their cells when washed with alcohol - staining purple

• P for positive and P for purple

Question 3

Describe the structure of gram negative bacteria and its staining colour?

Answer 3

Gram negative bacteria have cell walls a thinner peptidoglycan layer with an outer lipopolysaccharide layer.

On treatment with alcohol, the Gram negative cell walls
lose their outer lipopolysaccharide layer (stained purple) and the thin inner peptidoglycan layer is left exposed, this means that the crystal violet/iodine complexes are washed from the gram-negative cell along with the outer membrane - the peptidoglycan stains red with the counterstain safranin thus showing up red

Question 4

Name the shape classifications of bacteria

Answer 4

1. spirillum - corkscrew
2. bacillus - rod shaped i.e sausage
3. coccus - spherical

Question 5

An example of spirillum bacteria

Answer 5

• vibrio cholerae

- lyme borreliosis

Question 6

An example of bacillus bacteria

Answer 6

• E. coli

Question 7

An example of coccus bacteria

Answer 7

• streptococcus

- staphylococcus

Question 8

What is an obligate aerobe, and example?

Answer 8

obligate aerobes - bacteria requiring oxygen for metabolism, only metabolise in aerobic conditions

e.g mycobacterium tuberculosis

Question 9

What is an obligate anaerobe, and example?

Answer 9

obligate anaerobes - bacteria that can only survive in the absence of oxygen, oxygen is toxic to the organism

e.g clostridium spp.

Question 10

What is a facultative anaerobe, and example?

Answer 10

facultative anaerobes - can respire anaerobically if
they need to (if there is little or no oxygen available), but aerobic respiration is preferred

e.g e.coli

Question 11

What are aseptic techniques (also called sterile technique) used to prevent?

Answer 11

• prevent contamination of the environment by the microbes being handled
• prevent contamination of microbial cultures by unwanted microbes from the environment

Question 12

Appropriate methods for equipment and media to be sterilised before use?

Answer 12

• heat - use of an autoclave at a temperature of 121°C for 15 minutes under high pressure OR heating an inoculating loop in a Bunsen burner flame
• Irradiation - plastics which are heat labile (stable) such as petri dishes sterilised by gamma irradiation