Question

You are monitoring growth of a culture by taking 100 microliter samples from 1:1000 dilutions of the culture. At T0 plate count was 80 colonies, at T1(20) it was 120, at T2(40) is was 160. Give the CFU/ml of the culture for each time and indicate what was the doubling rate per hour of the organism.

Answer 1

CFU/ml = number of colonies / volume plaed in ml * dilution

For T(0) = number of colonies = 80

Volume plated = 100ul

dilution 1/1000

CFU/ml = 80 / 0.1 *1/1000 = 80 * 10 * 1000 = 8 *105 CFU/mlCFU

T0 = 8 *105 CFU/ml

For T1(20) = number of colonies = 120

Volume plated = 100ul

dilution 1/1000

CFU/ml = 120 / 0.1 *1/1000 = 120 * 10 * 1000 = 1.2 *106 CFU/ml

CFU T1 (20) = 1.2 *106 CFU/ml

For T2(40) = number of colonies = 160

Volume plated = 100ul

dilution 1/1000

CFU/ml = 120 / 0.1 *1/1000 = 160 * 10 * 1000 = 1.6 *106 CFU/ml

CFU T2 (40) = 1.6 *106 CFU/ml

Doubling time

G= t log2/ log Bt – log B0

t - the time is taken to increase population from B0 to Bt.

t = 40 minutes

B0 = 80

Bt = 160.

log 2 = 0.30

G = 40 * 0.3 / log160 - log80

log 160 = 2.2

log 80 = 1.9

= 40* 0.3 / 2.2 - 1.9

G= 40*0.3/0.3

G = 40 minutes.

or 40/60 = 0.66h.

So the douling time of bacteria is 0.66h