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?
A) 1.43 cells/ mL
B) 1.43 × 10² cells/ mL
C) 1.43 × 10³ cells/ mL
D) 1.43 × 10⁵ cells/ mL
E) Cannot be determined

Question

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? 
A) 1.43 cells/ mL
B) 1.43 × 10² cells/ mL
C) 1.43 × 10³ cells/ mL
D) 1.43 × 10⁵ cells/ mL
E) Cannot be determined

Quizplus · Accepted Answer

The original sample was diluted by a factor of 100 (1 mL in 99 mL), and then 0.1 mL of this dilution was plated. The dilution factor for the plated sample is 1000 (100 for the dilution and 10 for the 0.1 mL plated). Therefore, the concentration in the original sample is 143 colonies × 1000 = 1.43 × 10⁵ cells/mL.

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