Growth of bacteria

Question 1

Describe Miles & Misra method for measuring bacterial growth

Answer 1

1. Sample diluted in log steps
2. Set volume (50µl) is plated
3. As dilution gets lower, there are few enough bacteria that when liquid dries into media they stay separate & form separate colonies
4. Count colonies from 50µl & calculate how many would be in 1ml
5. Numbers expressed as colony forming units per ml (CFU/ml)

Question 2

Describe absorbance to measure bacterial growth in liquid culture

Answer 2

Increase in optical density over time
The more bacteria the greater the absorbance of light

Question 3

What are the phases of bacterial growth in liquid culture

Answer 3

1. Lag phase
2. Exponential phase
3. Stationary phase
4. Death phase

Question 4

Impact of pH on bacterial growth

Answer 4

Most bacteria are neutrophiles

Question 5

Impact of temperature on bacterial growth

Answer 5

Most infectious bacteria are mesophilic

Some adapted to animals with slightly different optima
Campylobacter grows best as 42ºc in birds

Storage in fridge slows bacterial growth but doesn’t stop it completely

Question 6

Impact of osmolarity on growth

Answer 6

Membrane of bacteria is semi-permeable

Accumulate/lose solutes to balance water potential across membrane

Prefer slight positive pressure

Question 7

Describe nutritional requirements of bacteria

Answer 7

Acquire nutrition from immediate environment (wound, intestine, blood etc.)

Bacteria can be:
Fastidious –> require specific supplements
Non-fastidious –> grow from basic chemicals

Most relevant organisms are chemoheterotrophs
Use organic chemicals as source of energy & carbon

Question 8

Methods of getting rid of bacteria

Answer 8

Sterilisation:
Eliminates/kills all forms of life
Heat, chemical, irradiation

Washing:
Doesn’t sterilise unless you use something that kills microorganisms
Can remove & reduce risk of contamination

Disinfection:
Damage in general manner by reacting with different targets (proteins, nucleic acids etc.)
Some can be used on patients (e.g. chlorohexidine)
Others are toxic (e.g. formaldehyde)
Selection must account for:
Not tainting food industry
Effectiveness in environment they will be used in
Effectiveness against organism

Question 9

How can you distinguish between bacteria

Answer 9

Morphology
Motility
Cell wall composition
Respiration
Growth on specific media
Haemolysis
Tolerance to compounds

Question 10

Describe how susceptibility to antimicrobials compounds can be used for selective media

Answer 10

Contain antimicrobial compounds which inhibit groups of bacteria you don’t want

Antimicrobials are compounds which inhibit bacterial growth

Allows those you want to isolate & grow

e.g. enteric bacteria resistant to bile salts so only they will grow

Question 11

Describe common bacterial tests & how they can support diagnosis

Answer 11

Haemolysis:
To assess ability of bacteria to lyse erythrocytes
Blood agar
Alpha, beta or gamma
Indicative of how virulent bacteria are

Catalase & oxidase test:
Illustrate how components relating to respiration can be present/absent

Coagulase test:
Identifies presence of bound coagulase or clumping factor

Urease test:
pH indicator test

Fermentation:
Indicates ability to ferment different sugars