Module 3: Bacterial Reproduction & Growth Curves

Question 1

What is generation time and what are the factors that affect it?

Answer 1

It is time for the bacterial population to double.
Factors:
1. Genetic control: determined by DNA
2. Available nutrients: some metabolize faster than others
3. Environmental: temp., O2, CO2

Question 2

What is in vivo and in vitro?

Answer 2

• In vivo: in the human body, generation times are generally longer due to host defences
• In vitro: in the lab, no host defences, faster generation times

Question 3

Explain the two types of bacterial counts.

Answer 3

1. Viable counts: counts only on live bacteria that have reproduced.
2. Total counts: both viable and dead bacteria.

Question 4

Explain McFarland standards.

Answer 4

Are prepared and represent a specific number of bacteria.
**1% Sulfuric acid plus 1.175% Barium chloride results in a white precipitate of Barium sulfate
Set of 10 tubes ranging from 0.5 McF to 9 McF
Examples:
0.5 McFarland = turbidity represents 1.5 x 108 bacteria/mL
1.0 McFarland = turbidity represents 3.0 x 108 bacteria/mL

Question 5

What do spectrophotometer or nephelometer’s do?

Answer 5

They can detect turbidity (cloudiness or haziness of a fluid) or light scattered by bacteria, which represents the bacterial concentration.

Question 6

Describe the “Lag Phase” of the bacterial growth curve.

Answer 6

The beginning of the curve:

• little or no increase in the number of cells
• actively taking in metabolites and nutrients

Question 7

Describe the “Log Phase” of the bacterial growth curve.

Answer 7

The second stage of the curve:

• cells dividing at a constant rate
• number of cells doubles in direct relationship to time (generation time)
• clear liquids turn cloudy and colonies appear on agar
• Log phase ends when cells stop dividing at a constant rate due to nutrient shortage, waste products, or both

Question 8

Why is the Log Phase of the curve important?

Answer 8

• Good cellular morphology, gram pos vs. gram neg
• Good bacterial motility
• Bacteria are most susceptible to antimicrobials

Question 9

Describe the “Stationary Phase” of the bacterial growth curve.

Answer 9

1. Total number of cells remains constant (no more nutrients)
2. All cells stop dividing OR Growth = death

Question 10

Describe the “Death Phase” of the bacterial growth curve.

Answer 10

• Exponential decline in viable cells
• Poor gram stains
• Involution forms
• *Best time to find spores!!
• Slide motility may be negative

Question 11

How do you calculate the colony forming units (CFU’s)/ ml of bacteria suspension?

Answer 11

10 Colony forming units X 1000ml/L = CFU/L