You are testing the growth parameters of a novel heterotroph

You are testing the growth parameters of a novel heterotrophic bacterial species you have isolated. To determine its optimal growth temperature, you will want to determine specific growth rates at various temperatures. To do so, you are using the spread plate count method to assess viable culture counts at particular times.

1. (4 pts) Your first task will be to enumerate the number of viable cells in your stock culture flask so you may use them for your experiment. To do so, you make a serial dilution (10 fold for each step) of the stock culture, and from each dilution tube in the series you plate 0.1 ml on a separate nutrient agar plate via the spread plate method, in duplicate. Here are your counts from the dilution series, as colony forming units (cfu, the name for individual colonies in a plate count):

Tube                Count (cfu)

1 10^-1               TNTC; TNTC

2 10^-2               322; 310

3 10^-3               71; 52

4 10^-4               9; 13

5 10^-5               1; 3

Using this information, calculate the viable concentration of cells in the stock flask. Remember to think about which plate counts are accurate and use the replicates appropriately.   

Solution

Answer: startting with obsrving the CFU count of each dilutions. the CFU count in dilution 2 is almost consistent in the two plates. therefore we take that as our base for further calculations.

1. we first take the average of the count = (322+310)/2 = 316 CFU

2. this number of colonies have come up after plating 0.1 ml of the culture

therefore the conecntration of viable cells = 316 cfu/0.1 ml

c\' = 316 cfu/ 0.1 ml

3. But this was a further diltion from the original stock therefore, the concentration of cells in the original stock is

C= c\' x Diltuion Factor

C= (316 cells /0.1 ml) x 100

C= 3.16 x 10⁵ cfu/mL

Therefore viable concentration of cells in the stock flask is 3.16 x 10⁵ cfu/mL