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 ? 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? E. What could be a possible reason for the difference between the counting chamber and the plate results?

I need the answer to quetion E, thank you.

Answer 1

Petroff Hausser chamber CFU method Direct counting method Indirect counting method Counts both viable and inviable cells Counts only viable cells Direct dilution is done Serial dilution is done No incubation time is required Incubation on a nutrient media is required Number of cells per uL can be counted easily Number of Colony forming units per mL is counted

Most important reason is point # 2 of the table.

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