MEASURING BACTERIAL GROWTH (WEEK 5)

What is generation time?

• the time required for a cell to divide

• time varies from one species to another and depends on environment

• ranges from 20 minutes to 3 hours

• cells double with each division

How do we determine bacterial growth?

• bacteria will grow by binary fission

• one cell will separate into two cells

• growth refers to reproduction, not an increase in cell size

What is the bacterial growth curve?

1. lag phase to prepare for growth without pop increase

2. log phase with log or expo increase in pop

3. stationary phase where microbial deaths balance new cell production

4. death phase where population decreases logarithmically

What is scientific notation?

• used for very large or small numbers

• 4,500,000,000 = 4.5×10^9

• 0.000000000045 = 4.5×10^-11

How do we count the number of bacteria in a sample?

• indirectly thru turbidity

• directly thru direct microscope count or plate counts

How do we measure turbidity?

• the more bacteria, the more turbid

• turbidity is measured using a spetcrophotometer

Why is turbidity advnatageous?

• results are obtained immediately

Why is turbidity disadvantageous?

• cannot be used for very dilute cultures

• dead cells are included

• need to construct a standard curve as each species may absorb light differently

What is a direct microscopic count?

• observing a very small known amount of liquid sample under the microscope using a slide to count organisms

Why is a direct microscope count advantageous?

• get results immediately

Why is a direct microscope count disadvantageous?

• cannot be used for very dilute cultures

• dead cells are included

• not for motile cells

What are plate counts?

• a small amount of bacterial culture is spread on a plate

• each individual bacteria will result in a visible colony

• # of colonies = # of bacteria

• measured as CFU or colony forming units

Why are plate counts advantageous?

• counts only live cells

• more accurate

Why are plate counts disadvantageous?

• incubations are needed so results are obtained after a few days

• dilutions are required

How do serial dilutions work?

1. known amount of sample is mixed with a diluent (usually water)

2. known amount of first dilution is then mixed with more diluent

3. repeated several times