3.4 Microbiology

Question 1

What are some ways bacteria can be distinguished from each other?

Answer 1

Size
Shape
Staining characteristics
Metabolic features
Antigenic features
Genetic features

Question 2

Shape of coccus bacteria

Answer 2

Spherical

Question 3

Shape of bacillus bacteria

Answer 3

Rod shaped

Question 4

Shape of spirillum bacteria

Answer 4

Spiral shaped

Question 5

Pleural terms for
• coccus
• bacillus
• spirillum

Answer 5

• cocci
• bacilli
• spirilla

Question 6

What is peptidoglycan?

Answer 6

Cell wall of a bacteria which is a mixture of polysaccharide and polypeptide

Question 7

What is the other word for peptidoglycan?

Answer 7

Murein

Question 8

Describe gram positive bacteria

Answer 8

• stains purple/violet
•thick peptidoglycan layer which retains the crystal violet
• no lipopolysaccharide layer
• not affected by lysozyme or penicillin

Question 9

Describe gram negative bacteria

Answer 9

• stains red
• thin peptidoglycan layer
• alcohol washes the crystal violet out
• counterstain with safranin
• extra lipopolysaccharide layer
• not affected by lysozyme or penicillin

Question 10

describe the gram stain procedure

Answer 10

1. Small sample of bacteria is transferred to glass microscope slide using an inoculating loop. Slide is passed through bunsen flame a few times to fix bacteria to the slide
2. Few drops of crystal violet is added and left for 30s
3. Rinse excess using water
4. Gram’s iodine is added for 1min to fix stain
5. Purple stained bacteria are gram-positive
6. Wash with alcohol for 30s to dissolve lipids in lipopolysaccharide layer and so exposing inner peptidoglycan layer
7. Re-stain using safranin which stains unstained bacteria red

Question 11

In what equipment is bacteria commonly cultured

Answer 11

Petri dish or in a flask

Question 12

Whats the most common medium that bacteria is cultured in

Answer 12

Agar

Question 13

What does agar contain in order to be a growth medium for bacteria?

Answer 13

Source of carbon
Source of nitrogen
Water
Vitamins and mineral salts
Suitable temperature
Suitable pH
Oxygen may or may not be required

Question 14

Why is bacteria cultured at 25 degrees Celsius not 37?

Answer 14

To avoid growing human pathogens as they can form colonies at 37 degrees Celsuis

Question 15

What are the three different oxygen requirements a bacteria can have?

Answer 15

Obligate aerobes
Facultative anaerobes
Obligate anaerobes

Question 16

What is are obligate aerobes?

Answer 16

Microbes that require oxygen to carry out metabolism and reproduction

Question 17

What are facultative anaerobes?

Answer 17

Microbes that carry out metabolism and reproduction better with oxygen but can grow without it

Question 18

What are obligate anaerobes?

Answer 18

Microbes that cannot survive in the presence of oxygen

Question 19

Obligate anaerobes in test tube

Answer 19

Collect at top of test tube

Question 20

Facultative anaerobes in test tube

Answer 20

Across the whole tube but more towards the top

Question 21

Obligate anaerobes in test tube

Answer 21

Towards bottom of test tube

Question 22

What is aseptic technique?

Answer 22

good laboratory practice that maintains sterile conditions and prevents contamination.

Question 23

What is the aim of aseptic technique

Answer 23

• prevent contamination of the environment by microbes
• prevent contamination of microbial cultures by unwanted microbes of the environment

Question 24

What is done before an experiment to carry out aseptic technique?

Answer 24

• wash hands
• sterilise benches using disinfectant
• tie hair back due to bunsen burner
• sterile equipment eg petri dish(heat), inoculating loop (heated until glowing red), other equipment(autoclave)

Question 25

What is done during serial dilution to carry out aseptic technique?

Answer 25

• lids of agar bottles held in crook of little finger - not on bench • necks of agar bottles are flamed on opening and closing • loops are sterilised and cooled before insertion to cultures and sterilised after • lids of petri dishes are tilted not lifted off when agar is poured in or while plate is being inoculated

Question 26

What is done after serial dilution to carry out aseptic technique?

Answer 26

• petri dishes are sealed with cross of tape to discourage growth of human pathogens • plates are incubated upside down at 25 degrees Celsius for 24 hours • plates are not reopened • plates are autoclaved after observations are made

Question 27

Total count method

Answer 27

Count of both live and dead bacteria

Question 28

Viable count method

Answer 28

Count of live bacterial colonies

Question 29

Methods in which total count method can be used

Answer 29

Haemocytometer slide Measure colour of broth in a colorimeter

Question 30

Describe how viable count technique is used to monitor population growth

Answer 30

• assume that one cell gives rise to one colony • use of aseptic technique • use of serial dilution - culture is diluted by ten-fold steps • 1cm3 of original sample added to 9cm3 of sterile water • these are mixed and the process is repeated • known volume of microorganisms are added to agar plates • these are incubated at 25 degrees C for 24-28 hours • number of colonies is counted and multiplied by dilution factor to calculate number of cells per cm3 in original sample

Question 31

How to know which plate to chose from serial dilution

Answer 31

One with the largest number of distinct colonies • clumping - cannot count accurately • too fee - inaccurate representation

Question 32

What is assumed during viable count technique and serial dilution

Answer 32

Each colony has arisen from a single bacterium

Question 33

Lag phase of bacterial growth

Answer 33

Slow population growth Bacterial cells take up water, protein synthesis, DNA/RNA synthesis, enzyme production

Question 34

Log phase of bacterial growth

Answer 34

Doubling per unit time by dividing asexually Optimum conditions Eventually carrying capacity is reached

Question 35

Stationary phase of bacterial growth

Answer 35

Death rate and reproduction rate equilibrate Limiting factors - nutrient depletion and waste accumulates

Question 36

Death phase of bacterial growth

Answer 36

Death rate exceeds birth rate Shortage of nutrients Lack oxygen Accumulation of toxic waste

Question 37

Phases in order of bacterial growth

Answer 37

Lag Log Stationary Death

Question 38

Phases of bacterial growth

Answer 38

Lag Log Stationary Death