Baceriology WS W8 Flashcards
Calculation of generation time
G = t / (3.3 lg b/B)
G = generation time
t = time interval in hours or minutes
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)
For example, what is the generation? Time of a bacterial population increases from 10,000 cells to 10,000,000 cells in four hours?
4 hours / (3.3 log10^7 / 10^4)
4 / (3.3 x 3) as 10/10=1 and 7-4=3
G = 24 minutes
Enumeration of Bacteria: you have stock solution of bacteria (10ml) and you want to find out the number of bacteria in this solution. You plate bacteria out in a dilution of 10-3 overnight on a nutrient plate (you plate out 0.1 ml). Next day you count 84 colonies on the nutrient plate. How many bacteria were in your stock solution?
84 colonies in 0.1ml = 840 in 1ml
We diluted to 10^-3 so need to x1000
840x1000=840,000 in 1ml
Need to x10 for 10ml = 8.4x10^6
Serial dilatations are used to dilate bacteria. How many 1:10 dilation steps will you have to do when you want to dilute 10,000 bacteria in 1mlof broth to 1 bacterium?
10,000 x 10^-1 = 1000
10,000 x 10^-2 = 100
10,000 x 10^-3 = 10
10,000 x 10^-4 = 1
= 4 dilutions
Enumeration of Bacteria: you have stock solution of bacteria (20ml) and you want to find out the number of bacteria in this solution. You plate bacteria out in a dilution of 10-6 overnight on a nutrient plate (you plate out 0.5 ml). Next day you count 3 colonies on the nutrient plate. How many bacteria were in your stock solution?
3 colonies in 0.5ml
6 colonies in 1ml
X10^-6 = 6,000,000
X10 = 60,000,000 in 10ml
X2 - 120,000,000 in 20ml
Do you want to test a bacterial suspension to see how many bacteria are there. You have 10 mL of the suspension and you take 1 mL. Dilation of 10^-8 and plate out 0.1 mL of the solution. Your assistant counts the bacteria and observes and report back to you that the amount of bacteria in your suspension was 20,000,000. Is this correct?
Expected colonies :
20,000,000 bacteria/ml
Dilution -> 20,000,000 x 10^8 = 0.2 bacteria/ml
Plated 0.1ml -> 0.2x0.1=0.02 which isnt visible is plausible
Plausible answer :
20 observed -> 20/0.1=200 bacteria/ml
200 x 10^8 = 20,000,000,000
Calculation: A bacterium has the generation time of 83min. After 24 hours of growth you count 3.5*105 bacteria. How many were present at timepoint 0 hours?
Nt = No x 2^n
n = total growth time / generation time
n = 24x60 / 83 = 17.35 generations
Nt = 3.5x10^5
No = Nt / 2^n
2^n = 2^17.35 = 163702.9
No = 3.5x10^5 / 163702.9 - 2.14
= approximately 2 bacteria
Calculation how long will it take 1 bacterium with a generation time of 20 minutes to divide into 1010 bacteria?
n=log2 (Nt/No)
n=log2 (10^10/1)
n=log2 (10^10)
n=10xlog2(10)
n=10x3.3219 = 33.219 generations
