1 Your first task will be to enumerate the number of viable

1. 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

For such calculations, the plate having number of colonies less than 200 should be selected.

Therefore, the plate corresponding to tube 3 (10^-3 dilution ) is selected.

Average cell counts for plate 3 = (71+52)/2 = 62

Viable concentration of cells in stock = [cfu (in plate 3) x dilution factor] / volume plated

= (62 x 10³)/0.1

= 6.2 x 10⁵