1. Bacterial Growth
November 2013

2. Bacterial Division Binary: one cell becomes two
Transverse: division plane across cell
Binary fission: parent cell enlarges, duplicates chromosomes, forms central transverse septum
Parent cell divides into two daughter cells

3. Binary Fission

4. Generation Time
Generation (doubling) time: time required for a complete fission cycle
1 parent  2 new daughter cells
Each new fission cycle increases the population by a factor of 2
1  2  4  8  16  32  64…

5. Generation Times
Average generation time: minutes under optimum conditions
Shortest generation time: minutes
Longer generation times: days
Mycobacterium leprae (causes Hansen’s disease) has a generation time of 10 to 30 days!
Leprosy (affects skin and peripheral nerves)
Unsure of how leprosy is transmitted, though we think it may be spread person to person in respiratory droplets

6. Generations # of cells 1 2 4 8 16 # of generations 3 Exponential Value
2^1
2^2
2^3
2^4

7. Exponential Growth
Exponential Growth: “uncontrolled” growth; occurs under ideal conditions and unlimited resources

9. Bacterial Growth Curves
Four phases:
Lag Phase: preparing for division
Exponential Growth/Logarithmic Phase: reproduction relationship to disease
This phase continues as long as cells have adequate nutrients and the environment is favorable
Stationary Growth Phase: population enters survival mode (cells stop growing or grow slowly); equalization
Death Phase: limiting factors intensify; cells are unable to multiply and begin to die at an exponential phase

10. Calculating Bacterial Growth
G (generation time) = time (minutes or hours)/n(number of generations)
G = t/n
B = number of bacteria at the beginning of a time interval
b = number of bacteria at the end of the time interval
n = number of generations (number of times the cell population doubles during the time interval)
b = B x 2n (This equation is an expression of growth by binary fission)

11. Practice
You perform a serial dilution and determine that the original number of cells in your sample was 12, 000. How many bacteria will be present in 12 hours if the generation time is 15 minutes (assume unlimited food and clean environment)?

12. Givens
You perform a serial dilution and determine that the original number of cells in your sample was 12, 000. How many bacteria will be present in 12 hours if the generation time is 15 minutes (assume unlimited food and clean environment)?
12 hours total time B= 12,000 bacteria
15 minutes/generation n= 48 generations in the given time

13. Answer
You perform a serial dilution and determine that the original number of cells in your sample was 12, 000. How many bacteria will be present in 12 hours if the generation time is 15 minutes (assume unlimited food and clean environment)?
12 hours total time B= 12,000 bacteria
15 minutes/generation n= 48 generations in the given time
b=B x 2^n
12,000 x 2^48 = 3.38 x 10^18 bacteria

14. Practice 2
2. Let’s say that flesh eating Strep. pyogenes divides every 10 minutes at body temperature. You fall down and scrape your knee and get infected with 5 Strep. pyogenes cells. After 4 hours, without medical intervention, how many bacteria will be ravaging your body? Let’s say that for every 1 million bacteria, a centimeter of flesh is consumed. After 4 hours, how much tissue would be lost? Are you still alive and would you want to be? (This problem is not fact based.)

15. Givens and Solving G (generation time) = 10 minutes
B (initial bacteria count) = 5
Time: 4 hours
N (number of generations) = 6 x 4 = 24
b= B x 2^n
b= 5 x 2^24
b= 83,886,080 bacteria
For every 1 million bacteria, a cm of flesh is consumed
Here, 83.9 cm of flesh would be consumed