Question

You have a flask from an overnight culture and take 1 mL from this flask and put it into 9 mL of PBS to dilute it (tube 1), you then take 1 mL from tube 1 and put that in a tube with 9 mL of PBS to make tube 2. You take 1 mL from tube 2 and put it in a tube with 9 mL of PBS to make tube 3. You take 1 mL from tube 3 and add it to tube 4 with 9 mL of PBS. Finally, you take 0.1 mL from tube 4 and plate it on an agar plate. The next day, you count 218 colonies on the plate. What was the concentration of bacteria (in CFU/mL) in the original overnight flask?

Answer 1

in tube 1 the dilution of the culture will be 1:10 ( 1 ml of cluture in 10 ml of the suspension)

in tube 2 the dilution of the culture will be 1:100

in tube 3the dilution of the culture will be 1:1000

in tube 4 the dilution of the culture will be 1:10000     The culture has been diluted in the ratio 1:10 serially 4 times, then the dilution factor is = 10X10X10X10= 10⁴

CFU/mL = (no. of colonies x dilution factor) / volume of culture plate= (218 X 10⁴)/0.1 ml= 2.18 X 10 ⁷ CFU/mL