You want to quantify the bacteria in a culture by diluting and plating for colon

ID: 145877 • Letter: Y

Question

You want to quantify the bacteria in a culture by diluting and plating for colony forming units (cfus).

You take 100 microL from your culture and add it to 900 microL of medium in Tube A.

You take 100 microL from Tube A and add it to 900 microL of medium in Tube B.

You take 10 microL from Tube B and add it to 990 microL of medium in Tube C.

You take 200 microL from Tube C and spread all of it on an agar plate.

45 colonies grow up on that plate.

What is the concentration per mL of bacteria in your original culture?

Ans. 1000ul=1ml

So, if we take 100ul from culture and added it to medium of 900ul=100/100+900=1/10 dilution

And further when we transfer 100ul from tube A to 900ul in tube B=100/100+900=1/10 dilution

And now 10ul from tube B to 990ul in tube C=10/10+990=1/100 dilution

Total dilution factor =1/10*1/10*1/100=104

Now,200ul or 0.2ml is plated from tube C to the agar plate and 45 colonies grow on the plate.

Cfu/ml=no. Of colonies x dilution factor /volume of culture plated

=45 x 104/0.2=2.25 x 106 cfu/ml

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.