Question

=4. Let's practice a calculation of CFU/g, one that will mimic your owncalculation in the lab this week. Suppose a student weighed 1 g of soil andcreated a slurry using 10 mL of water for a concentration of 1g/10mL = 0.1g/ml. She performed a 1:2 serial dilution - 0.5 mL slurry into 0.5 mL water - suchthat the concentrations of her samples were as follows:Tube 1: 0.05 g/mLTube 2: 0.025 g/mLTube 3 0.0125 g/mLShe plated 100 ul of tube 3 onto an agar dish and counted 100 colonies on theplate. What is the CFU/g?

Answer 1

The student plated 100 uL of tube 3. The concentration of tube 3 is 0.0125 g/mL. She counted 100 CFU on the plate. We need the answer in CFU/g, so the first step to solve the problem is to determine the mass of the sample that she plated.

The concentration in this case is defined like:

concentration = mass of sample in g/(volume of sample in mL)

So we can obtain the mass of the sample in g by:

mass of sample in g = concentration * volume of sample in mL

The concentration is g/mL and she took 100 uL. Let's convert those uL into mL. We know that there are 1000 uL in 1 mL.

1000 uL = 1 mL

volume of sample in mL = 100 uL * 1 mL/(1000 uL)

volume of sample in mL = 0.100 mL

We know the concentration of tube 3 and the volume of the sample in mL we can find the mass that the student plated.

mass of sample in g = concentration * volume of sample in mL

mass of sample in g = 0.0125 g/mL * 0.100 mL

mass of sample in g = 0.00125 g

So the student took 100 uL of tube 3 or 0.00125 g of that sample. In that sample she counted 100 colonies. So the CFU/g is:

CFU/g = 100 CFU/(0.00125 g)

CFU/g = 80000 CFU/g

Answer: The CFU/g is 80000