Question

Question 10. Bacterial density in a counting chamber A scientist needs to calculate the density of a bacterial culture in order to set up an experiment, but the laboratory spectrophotometer is broken and the only alternative is to use a counting chamber that she finds in a drawer of old equipment. The counting chamber has a glass to coverslip height of 0.02 mm and the instructions say that each square of the chamber measures 10 um x 10 um. She counts bacteria in 5 squares. The 5 squares contain 264, 287, 240, 267 and 251 bacteria. A. What is the volume of liquid corresponding to one square in um? B. What is the volume of liquid corresponding to one square in ul? C. What is the average density of bacteria in cfu/ul? Because the scientist isn't certain how accurate this counting chamber is, she makes a 10 serial dilution and plates out 0.5 ml of it onto a nutrient agar plate, to check whether the number of resulting colonies corresponds to what she expects. After incubation, there are 33 colonies on the plate. D. What density of bacteria, in cfu/ml, does this result give?

I need the answer to question D, thank you.

Answer 1

Total volume = 0.000002 × 5 = 10^-5 uL

Total # cells = 264 + 287 + 240 + 267 + 251 = 1309

10^-5 uL = 1309 cells

1 uL = 1309 / 10^-5 = 1309 × 10^5 = 1.309 × 10^8 cells per uL

1 uL = 10^-3 mL

10^-3 mL = 1.309 × 10^8 cells

1 mL = 1.309 × 10^8 / 10^-3 = 1.309 × 10^11 cells per mL

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