Question

1. Use the following example of a series of dilutions and plating to answer the problems below. 1 ml 1 ml 1 ml 1 mL 1 mL 9 mL 9 mL 9 ml 9 ml Sample of E.coli 1:10 1:100 1:1000 1:10,000 1:100,000 10- 10- 60 1 mL 1 mL Tim Time O O O O O Colony counts: Colony counts: Toon Too many to count 1354 1354 290 290 75 75 10 10 A. What is the original cell density (O.C.D) of the sample of E. coli if using the count from plate D? Must have units. B. How many plates have colony counts within the accurate range? C. What is the O.C.D. if using the count from plate C? Must have units.

microbiology

Answer 1

a) The first thing that you must do is to calculate the dilution factor. For this, multiply each 10 fold dilution until reach the plate D Dilution factor = 1/10 x 1/10 x 1/10 x 1/10 = 1/10.000 = 10-4 Now use the following equation: Number of colonies 75 CFU/mL = = 750000 CFU/ml Dilution Factor x Volume plated 10-4 x 1 mL b) Theoretically, the range of accurate count must be between 60 to 160 colonies per plate. So, only one plate has a accurate colonies count Number of colonies 290 CFU/mL = = 290000 CFU/ml Dilution Factor x Volume plated 10-3x1 ml