Question

Below is a growth data of a microorganism cultured in a flask containing a liquid nutrient medium. Samples were taken hourly and number of cells counted using a counting chamber. Data is provided in cells per mL.

Draw a growth curve in Excel and use it to calculate generation time. The answer must be provided in minutes.

Do not discard your growth curve as you will need to upload it after submitting your answer to this question. Make sure you growth curve has the X and Y axes labelled and add the appropriate units.

Time	Cells per mL
0	4.90E+05
1	5.00E+05
2	2.20E+06
3	8.20E+06
4	3.20E+07
5	1.70E+08
6	7.30E+08
7	2.88E+09
8	3.00E+09
9	3.40E+09

Note: 3.00e+06 is equivalent to 3.00 x 10⁶

Answer 1

Solution

Tabular representation of data

To plot the growth curve, we need to construct the following table:

Time (hr)	Time (min)	Cells/ml	Log no. of cells/ml
0	0	4.90E+05	5.69019608
1	60	5.00E+05	5.698970004
2	120	2.20E+06	6.342422681
3	180	8.20E+06	6.913813852
4	240	3.20E+07	7.505149978
5	300	1.70E+08	8.230448921
6	360	7.30E+08	8.86332286
7	420	2.88E+09	9.459392488
8	480	3.00E+09	9.477121255
9	540	3.40E+09	9.531478917

Graph Plot

The plotted graph has Log no. of cells/ml values as Y-xis and Time (in min) as X-axis.

Calculation of generation time

The exponential pgase of the growth curve follows geometric progression. Considering any two points during this phase can provide information needed for calculation of the generation time.

We consider two time points, ti=180 mins and t0=120 mins

(i) G = generation time = t/n

where, t (time) = ti (final)- t0 (initial) = 180-120 = 60 mins

n (generation number) = 3.3 log b (cell/ml at ti) / B (cells/ml at to) = 3.3 log (b/B)

= 3.3 [log b - log B]

= 3.3 [log (8.2 x 106) - log (2.2 x 106)]

= 3.3 ( 6.91 - 6.34)

= 3.3 x 0.57 = 1.88 2

This organism completes 2 generations in 60 mins.

Generation time (time taken for completion of 1 generation) = t/n = 60/2 = 30 mins

Therefore, this organism takes 30 mins to complete 1 generation i.e, doubles its population in 30 mins.