You are determining the number of colony forming units (CFUs) for Susan, who has a urinary tract infection (UTI). You added 1 mL of Susan's urine sample to 99 mL of diluent, and spread 0.1 mL of that dilution on a culture medium. After appropriate incubation, you count 143 colonies on the plate. What was the concentration of bacteria in the original urine sample?

Question

Quizplus · Accepted Answer

To determine the concentration of bacteria in the original urine sample, we need to do the following calculations:

1. We know that 0.1 mL of the 1:1000 dilution was plated on the culture medium.
2. We observed 143 colonies on the plate.
3. Therefore, there were 143 CFUs in 0.1 mL of the 1:1000 dilution.
4. To determine the concentration of bacteria in the original sample, we need to multiply the number of CFUs by the dilution factor.
5. The dilution factor for this sample is 1:1000.
6. To convert the 0.1 mL of the diluted sample to the original volume of the sample, we multiply by the reciprocal of the dilution factor:
      0.1 mL x (1/1000) = 0.0001 L
7. We can now calculate the concentration of bacteria in the original sample:
      CFUs / volume of the diluted sample x reciprocal of the dilution factor
      143 CFUs / 0.0001 L x (1/1000)
      = 1.43 x 10^5 CFUs/L or cells/mL

Therefore, the correct answer is D) 1.43 x 10^5 cells/mL.

You Are Determining the Number of Colony Forming Units (CFUs)